Add Rake task to create new Lexers

`rake "lex[:language]"` will create all necessary files
Merge remote-tracking branch 'coffeejunk/rake_task'
Add boilerplate for LLVM

- filename guessing
- source guessing
- mimetype
Lower confidence to 0.1

https://github.com/jayferd/rouge/pull/63#discussion_r3251587
Implement LLVM Lexer

closes #22