Add GRETL Lexer

[khwilson]

Per #3, we would like to support syntax highlighting for all the relevant languages here. https://github.com/rouge-ruby/rouge doesn't have a Stata or GRETL lexer. I've written a Stata one, but don't want to fix up all the test issues to get it through Rouge's process, so for now it lives in the _plugins directory.

If you'd like to add a GRETL lexer, feel free to follow the same process as for the Stata lexer. :)