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
Handle whitespace as 'Text' and not 'Text.Whitespace.

This commit removes many whitespace nodes which aligns
Elixir code correctly.
Remove stray mixin.
No need to escape regexp's as Ruby does it for us.
Make long Regexs more readable.
Lex double colon as Punctuation.
Merge remote-tracking branch 'pr/84'
add a demo for elixir