Mark the end of a do block

[niklasmoeller]

Indicate which "end" statement belongs to the "do" statement under the cursor.

I used autocomplete-elixir, which supported that. Since it's deprecated, I would love to have that feature within atom-elixir.

This is how it looked like in autocomplete-elixir:

[jnmandal]

I abstracted the logic for this into a standalone package I am using.. I can open a PR for it on this project