Add support for the Alloy language

Adds support for the Alloy language (see http://alloy.mit.edu/). Alloy is a language for describing structures and a tool for exploring them. It has been used in a wide range of [applications](http://alloy.mit.edu/alloy/applications.html "Alloy Applications") from finding holes in security mechanisms to designing telephone switching networks.

Some repositories using this language:

https://github.com/eskang/500lines
https://github.com/gchpaco/xacml
https://github.com/jonnybest/benchmark-results
https://github.com/juodaspaulius/BehavioralClafer
https://github.com/jschear/cs195y-case-studies

And the list goes on (an advanced search for "model extension:als" [shows 459 code results](https://github.com/search?q=model+extension%3Aals&type=Code "Code results")).

I'll be sending a pull request to Pygments for an Alloy lexer, but since it might take some time for that change to be integrated (and for it to appear on pygments.rb) I temporarily set `lexer` to `Text only` for Alloy.