Add support for .editorconfig file extension

[Alhadis]

Description

There appears to be ample usage of .editorconfig as a legitimate file extension, rather than a complete filename. This PR adds it as an extension to EditorConfig's language entry.

Sidenotes

My initial approach was this…

diff --git a/lib/linguist/languages.yml b/lib/linguist/languages.yml
index ac5fe47d..5e7d9ea1 100644
--- a/lib/linguist/languages.yml
+++ b/lib/linguist/languages.yml
@@ -1675,8 +1675,6 @@ EditorConfig:
   group: INI
-  filenames:
+  extensions:
   - ".editorconfig"
   aliases:
   - editor-config
   ace_mode: ini

… however, test_samples.rb got confused by the EditorConfig sample named .editorconfig (which ostensibly doesn't equate to a filename containing only an extension):

  1) Failure:
TestSamples#test_ext_or_shebang [/Users/Alhadis/Forks/GitHub-Linguist/test/test_samples.rb:40]:
/Users/Alhadis/Forks/GitHub-Linguist/samples/EditorConfig/.editorconfig should have a file extension or a shebang, maybe it belongs in filenames/ subdir

1654 runs, 33562 assertions, 1 failures, 0 errors, 0 skips

@lildude, please let me know if this is correct behaviour; if so, I'll stick with the less-than-D.R.Y approach I resorted to for this PR. 😉

Checklist

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories:
    Code Search yields ~39,000 results for path:**/?*.editorconfig.
  • I have included real-world usage samples:
    NB: The latter sample was edited to eliminate a lot of repetitive and excessive data.
    • [ASP.NET.EditorConfig]: Source | MIT-licensed
    • [IntelliJ.editorconfig]: Source | Apache-2.0
  • I have included a change to the heuristics

[lildude]

@lildude, please let me know if this is correct behaviour; if so, I'll stick with the less-than-D.R.Y approach I resorted to for this PR. 😉

Yup, I think it is. We already have other languages doing this.