Support Agda and Literate Agda

[Agda](http://wiki.portal.chalmers.se/agda/pmwiki.php?n=Main.HomePage) is a dependently type programming language and proof assistant, which is quite popular in the type theory community and is used [a lot](https://github.com/search?q=extension%3Aagda&type=Code&ref=advsearch&l=) on GitHub, for example in the [HoTT](https://github.com/HoTT/HoTT-Agda) cutting edge math research project

There is another [pull request](https://github.com/github/linguist/pull/252) adding support for Agda but that has been closed because of the lack of a Pygments lexer. Now, I've implemented a lexer which is in the main branch of pygments but hasn't landed in a release yet. I'm creating a new pull request because apparently the old one can't be reopened. Are there any additional steps I have to take because the lexer isn't in a released version of either pygments or pygments.rb?