Simplify directive combinator (module attributes)

[mracos]

Before

After

Things that I already mapped on #6 and this PR address:

• [x] Fix the combinator for module attributes (e.g. -module, -import)

Changes that are unrelated to the scope of this PR and can be done in another one:

• Match on escaped characters instead of literal ones, e.g. ?\b instead of ?b: 9e89d1345fd3482b910e932942f44b089e0ffa8b

I've been based all my changes on the "semi-official" erlang/spec, mostly the Grammar Appendix.

[tmbb]

Do the tests pass? I'm on a PC without elixir so I can't test them. If they pass, I'll merge

[mracos]

@tmbb Yep