Make it easier to look users up

[jimmycuadra]

Currently, users can be looked up only by ID or name. ID is annoying for chat services where you may not know the ID offhand, e.g. HipChat or IRC. Name is problematic because it has to be typed exactly, which sometimes requires quoting the name, and sometimes won't match exactly what's stored in Redis.

There should be a fuzzy user look up method that makes it easy for plugins to look users up by a key that could be ID, name, mention name, or a fuzzy match of name.