Add EditorConfig file and language sub-entry

[Alhadis]

This is a two-part PR related to the EditorConfig format, which I'll describe in ascending order of importance:

1. Language sub-entry

Recently, I took over maintainer duties for Atom's editorconfig package, which included a grammar for .editorconfig files. Naturally, I couldn't help rewriting it to fix various errors in Atom's highlighting — I figure if there's a dedicated grammar for EditorConfig files, we might as well use it on GitHub too.

I understand this might be pushing it, since the INI grammar provides decent-looking highlighting:

• Before
• After

Visible differences to highlighting include file-globbing operators (like ** and *) and tokenisation of bracket-pairs embedded in [{section,titles}], which are EditorConfig-specific facets.

2. Linguist's own .editorconfig file

I've added an EditorConfig for Linguist, so contributors with editors configured differently to GitHub's preferences (like myself 😝) can edit files without configuring their program to use a different tabstop-width or indentation style.

I should stress the addition of this file will not affect automation or tests in any way. It's strictly for the benefit of contributors, as it only changes what settings their editor of choice uses when hacking on Linguist.

Virtually every modern text-editor supports .editorconfig files either out-of-the-box or with a community-maintained plugin. Even GitHub acknowledges these files when determining what tab-width to use when displaying source code (albeit with various shortcomings, each of which I've reported at least twice now...)

Checklist:

• I am adding a new language.
  • The extension of the new language is used in hundreds of repositories on github.com.
    • .editorconfig: ~5,298,918 results
  • I have included a real-world usage sample for all extensions added in this PR:
    • It's simply INI's sample for .editorconfig
  • I have included a syntax highlighting grammar:
    • MIT license
    • Before / After

[Alhadis]

Ugh, sorry about the switch-a-roo of review-requests.... Internet (and GitHub) are lagging... 😢

[pchaigno]

I didn't know EditorConfig. Looks nice!

I understand this might be pushing it, since the INI grammar provides decent-looking highlighting:

It's probably for the best, the INI grammar is not receiving a lot of attention.

[Alhadis]

While I'm reminded, I should confess I had a dream a few weeks ago where I got in trouble with GitHub staff for accidentally committing a Ruby file to Linguist with tabs in it. Seriously.

Starting to think I need help...