For consistency, make Completion Handlers be implemented the same way Context Handlers are.
Previously when we added the Context parameter, we left the original signature in place and simply tacked , c on the end.
This made migration easier than changing all completion handlers bodies.
But since 0.5.0 is a big migration already, let's just get it over with.
See changes to Bukkit default completions and the example plugin for migration example.
Aikar