Pull in file extensions from Pygments

I read all the file extensions that Pygments knows about, mapped them
back to the appropriate Linguist::Language, and added them to the
languages.yaml file.
Add language aliases from Pygments

I went through all the aliases Pygments supports and removed the ones
that could already be used to find a Linguist::Language. Then I found
the Pygments::Lexer associated with each alias and found an associated
Linguist::Language for it (looking for a language with the same name as
the lexer, or by looking for the first langauge that uses that lexer).
Then I added the alias to the language's alias list.
Make Language.[] and Language.find_by(alias|name) ignore case

This makes it easier to find a Linguist::Language for a fenced code
block in Markdown, where language names are often specified in
lowercase.
Merge pull request #1675 from github/more-aliases

Pull in aliases and file extensions from Pygments