Add ._js & ._coffee extensions for Streamline.js.

Streamline.js by Bruno Jouhier (@Sage) is a library/tool that lets you write
synchronous-looking JavaScript/CoffeeScript, and have it compile to Node-style
asynchronous JavasScript/CoffeeScript. (It's amazing and super elegant.)

https://github.com/Sage/streamlinejs

From the start, Streamline was designed to be exactly JavaScript/CoffeeScript,
so that it works with existing editors and tools. No new keywords or syntax --
it simply reserves `_` as a function parameter name.

Streamline files have thus kept `.js` and `.coffee` as extensions, so that no
changes need to be made to tools; Streamline has just used the convention of
ending file basenames with `_`, like `foo_.js` or `foo_.coffee`.

Unfortunately, to get the most optimal Node.js integration, with on-the-fly
compilation (like CoffeeScript), files now need to have their own extension.
So Bruno has decided to make `._js` and `._coffee` the extensions of choice.

The good news is that the language is still exactly JavaScript/CoffeeScript;
it's only the extension that's changed. It's simple to add these extensions
into our editors, but I'm wondering if we can add this to GitHub too. =)

This change is super simple and shouldn't affect any other language, so I hope
you guys are open to it. Thanks for your consideration!

test/test_language.rb

@@ -252,6 +252,11 @@ class TestLanguage < Test::Unit::TestCase
     assert_equal Language['PHP'], Language.find_by_extension('php5')
     assert_equal Language['PowerShell'], Language.find_by_extension('psm1')
     assert_equal Language['PowerShell'], Language.find_by_extension('ps1')
+
+    # Aliases for Streamline.js ( https://github.com/Sage/streamlinejs )
+    assert_equal Language['JavaScript'], Language.find_by_extension('_js')
+    assert_equal Language['CoffeeScript'], Language.find_by_extension('_coffee')
+
     assert_nil Language.find_by_extension('.nkt')
   end