Add more language aliases

c++             -> cpp
coffee          -> coffeescript
coffee-script   -> coffeescript
cs              -> csharp
js              -> javascript
json            -> javascript
py              -> python
rb              -> ruby
sh              -> bash

All alises except "json" are based on Atom's mapping.
https://github.com/atom/language-gfm/blob/master/grammars/gfm.cson

JSON is not JavaScript, strictly speaking, but they are similar
enough and is offered for convenience.

Close #56