Massive Rewrite and Update

[rbuchberger]

Forgive the chaotic commit log.

I'll leave this pull request up for a little while for input, and then merge it into master if there are no objections!

Motivation

Most importantly, I want to thank Rob Wierzbowski and all the original contributors. While the
changes in this version are very extensive, its roots are still based in the purpose and approach of
the original.

Jekyll Picture Tag's last update was in October 2015. While the plugin grew stale, its application
certainly didn't; doing responsive images correctly is still involved, tedious, and tricky, and
there are still a few people out there building Jekyll sites.

Browser adoption of picture, source, srcset, and sizes is far better when this plugin was
first written. Firefox gained support in May of 2015, Safari in October of 2015, and Edge in october
of 2017. As of November 2018, all major, modern browsers have support and caniuse reports global
compatibility at 88%.

PictureTag's previous versions supported a few javascript solutions, but they didn't do a great job
supporting stock HTML (which, given browser suppport, was the most appropriate decision at the time).
That situation has now changed; standard HTML should be considered its core functionality, while
maintaining the ability to support javascript-based solutions in the future.

Goals

These were the objectives of the rewrite:

• Do not stray beyond the original objective of this plugin: Automate the creation of responsive
  image files and their associated markup. In other words, do one thing well.
• Maintain backwards compatibility with the old liquid tag syntax, to save website maintainers from
  needing changes made across their entire website.
• Streamline the configuration process, ideally make all configuration optional. In the absence of
  explicit instruction, do something reasonable.
• Enable multiple image formats (specifically webp). Generalize and modularize the code which
  generates output markup formats, to simplify the addition of new ones.
• Break up the original plugin, which consists of ~250 (very long) lines split across only 3
  functions, into something more manageable.

Changes

• The default output formats are bone-stock HTML (both <picture> and <img>. Picturefill, as of
  version 3, no longer requires special markup, so it remains compatible. Other javascript image
  solutions are supported with the data_ selection of markup formats. Interchange support is
  removed, though adding it again is not difficult if there is demand for it.
• There are now 2 basic markup formats to choose from: <source> tags within a <picture>, and a
  single <img> tag. If markup is set to 'auto', or if it is not set at all, the plugin will
  automatically determine which is most appropriate. These formats also have data_ counterparts
  (i.e. data_auto), which accomplish the same thing except setting respective data-attributes
  to allow for lazy loading and such.
• There are also 2 srcset formats: one which supplies image width, and another which supplies
  multiples (pixel ratios) of the base size.
• Source Keys are replaced by media query presets, which can also be used to create the 'sizes'
  attribute.
• Jekyll Picture Tag no longer accepts height arguments, and will no longer crop images for you.
  Aspect ratio will always be maintained.
• Multiple image widths are now supported, which will be used to create a corresponding srcset.
• Multiple image formats are now possible, including webp.
• PictureTag can now generate sizes attributes.
• Configuration takes a very different format. It should be simpler to write your config than the
  old version, and migrating to it should not be difficult. Instead of creating individual source
  keys, you supply an array of widths you'd like to construct. You can also supply an array (yaml
  sequence) of formats, including 'original'.
• Only global settings are placed in _config.yml. Presets are moved to _data/picture.yml.

Stats:

Keep in mind that the new version adheres to an 80 character line limit, while the old version had no line limit at all. (There were some whoppers in there!)

rbuchberger@rikki-lenovo ~/development/jekyll-picture-tag                       [11:06:03] 
> $ cloc ./                                                                    [±master ✓]
      15 text files.
      15 unique files.                              
       7 files ignored.

github.com/AlDanial/cloc v 1.76  T=1.02 s (8.8 files/s, 621.8 lines/s)
--------------------------------------------------------------------------------
Language                      files          blank        comment           code
--------------------------------------------------------------------------------
Ruby                              3             51             54            176
Markdown                          2             91              0            139
HTML                              2              9              5             48
YAML                              1              2              4             47
Bourne Again Shell                1              2              1              4
--------------------------------------------------------------------------------
SUM:                              9            155             64            414
--------------------------------------------------------------------------------
                                                                                               
rbuchberger@rikki-lenovo ~/development/jekyll-picture-tag                       [11:06:07] 
> $ git checkout refactor                                                      [±master ✓]
Switched to branch 'refactor'
                                                                                               
rbuchberger@rikki-lenovo ~/development/jekyll-picture-tag                       [11:06:12] 
> $ cloc ./                                                                  [±refactor ✓]
      38 text files.
      38 unique files.                              
       7 files ignored.

github.com/AlDanial/cloc v 1.76  T=1.02 s (31.5 files/s, 1779.1 lines/s)
--------------------------------------------------------------------------------
Language                      files          blank        comment           code
--------------------------------------------------------------------------------
Ruby                             24            170            170            653
Markdown                          3            189              0            523
YAML                              4             13             33             49
Bourne Again Shell                1              2              1              4
--------------------------------------------------------------------------------
SUM:                             32            374            204           1229
--------------------------------------------------------------------------------

Issues Fixed, implemented, or rendered irrelevant:

close #75, close #71, close #68, close #63, close #54, close #52, close #51,
close #26, close #24, close #19, close #13, close #12, close #7, close #6,
close #5