completions: behaviour callbacks

[mhanberg]

Problem

There is not autocomplete for callbacks for the behaviours of the current module.

Solution

When the cursor is in a situation like def | or defp |, the completions should show callback names as candidates.

So for a GenServer, it would autocomplete handle_call, init, handle_cast, etc.

[fuelen]

just came to create this issue :)

[nicolasferraro]

I'd love to tackle this one if it's up for grabs!

[NJichev]

Go for it, I don't think anyone's working on this one.

[mhanberg]

@nicolasferraro id hold off actually.