Add Erlang lexer implementation

If you merge this pull request and @adambard updates your gem then code at http://learnxinyminutes.com/docs/erlang/ will be highlighted!

Note: taken from https://bitbucket.org/birkenfeld/pygments-main/src/7304e4759ae65343d89a51359ca538912519cc31/pygments/lexers/functional.py?at=default#cl-1617

See #30
See https://github.com/adambard/learnxinyminutes-docs/pull/75#issuecomment-20290433