Implement lexer for Elixir language.

This is a direct port from pygments.
See https://bitbucket.org/birkenfeld/pygments-main/src/7304e4759ae65343d89a51359ca538912519cc31/pygments/lexers/functional.py?at=default#cl-2362

## The visual test

![elixir](https://f.cloud.github.com/assets/28908/781632/796b5fba-ea28-11e2-8d2d-88f6a9b00e8f.png)