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
llvm: remove the source-guessing spec
Merge branch 'llvm'
v0.3.0
Move Sass::COMMON to SASS_COMMON

Fixes #64.  I'm not that happy with this solution, but it works for now
v0.3.1
hotfix: copy the identifier variable #iamanidiot
v0.3.2
add a note about middleman-rouge to the README
Added test for prompt support in Shell lexer
Added prompt support in Shell lexer
Added test to not confuse prompt and variable in Shell lexer
Force prompt to have at least one space after the dollar sign in Shell lexer (to avoid confusion between a prompt and a variable)
Added CSS3 attributes to CSS Lexer
Fixed a bug in detecting comments in Shell Lexer
Added support for different types of prompts ($, >, #, %) in Shell Lexer