start of a Lua lexer

[ported from Pygments](https://bitbucket.org/birkenfeld/pygments-main/src/157c9feaccb86ac52da3d6b234cad92c85ee419c/pygments/lexers/agile.py?at=default#cl-1016), closes #18

Not quite done:

* I wasn't sure how to do some things with the string escaping section
* There was also some odd stuff with the function name state
* [builtins rake task and such](https://bitbucket.org/birkenfeld/pygments-main/src/157c9feaccb8/pygments/lexers/_luabuiltins.py?at=default)