Capture defp? as meta to get different coloring

Capture `def` and `defp` as meta in the language syntax to get different coloring. This is consistent with
how other languages such as Erlang and Python for instance color function names in their definition blocks in Sublime.

Note: I'm using a rather simplistic theme, so this might/might not affect other people.
Let's change it to account for at least `def(macro)?p?` and the identifier rule should be `[a-z_]\w*` (instead of what we have today).