Add environment variables for macro debugging flags (#1628)

[beckjake]

Fixes #1628 by adding DBT_MACRO_DEBUGGING environment variable support, by default no linecache is set.

While I'm in here adding debugger environment variables, I added two useful tricks I've been using locally so I don't have to keep manually patching them in:

• when the environment variable is set to write, write the files to disk so the debugging experience is more pleasant. This writes a lot of files and doesn't clean them up! But boy is it helpful sometimes, just to see where you are.
• when the environment variable is set, a special entry is inserted into the default context: {{ debug() }} will drop you into a debugger at the current line in the compiled macro. This is pretty helpful when you're debugging low-level macro weirdness and don't want to step through a million internal jinja functions just to see where you are.

$ cat models/x.sql
{{ debug() }}
select * from {{ source('src', 'seedtable') }}

With DBT_MACRO_DEBUGGING set:

$ DBT_MACRO_DEBUGGING=true dbt compile
Running with dbt=0.14.0
> /Users/jake/src/fishtown/projects/debug/bfb3707e2a6c4e27ff565365(14)root()

ipdb> l 9,12
      9     l_0_debug = resolve('debug')
     10     l_0_source = resolve('source')
     11     pass
     12     yield '%s\nselect * from %s' % (

With DBT_MACRO_DEBUGGING=write set:

$ DBT_MACRO_DEBUGGING=write dbt compile
Running with dbt=0.14.0
> /var/folders/31/mrzqbbtd3rn4hmgbhrtkfyxm0000gn/T/dbt-macro-compiled-cxvhhgu7.py(14)root()
     13         environment.call(context, (undefined(name='debug') if l_0_debug is missing else l_0_debug)),
---> 14         environment.call(context, (undefined(name='source') if l_0_source is missing else l_0_source), 'src', 'seedtable'),
     15     )

ipdb> l 9,12
      9     l_0_debug = resolve('debug')
     10     l_0_source = resolve('source')
     11     pass
     12     yield '%s\nselect * from %s' % (

The extra context really shines in longer macro/function call chains when you're walking up and down the stack trying to figure out why all your ref() calls started returning None after your minor tweak to the parser or whatever.

[drewbanin]

This is super cool! Love that you can add a debugger breakpoint :)