Elixir's lexer updates

The Elixir lexer has evolved over time.

This PR tries to pick up its changes. Please see individual commits.

Sadly, I could not cover all changes :/

In near future we might use [pygments' example file](https://bitbucket.org/birkenfeld/pygments-main/raw/b9e4312615752d4a4eba87ffde8fb97f54a27ab0/tests/examplefiles/example_elixir.ex) which exposes Elixir's changes pretty nicely.

What do you think?