Add the ability to register chat commands that execute on client.

Adds the ability to register chat commands that only execute on the client. Works with autocomplete.

Client commands are gray when shown in the autocomplete list (when you press tab)

I also added a clear chat command just as an example.

![clearchat](https://f.cloud.github.com/assets/2662937/745244/b8f1cf72-e40e-11e2-9b95-c76847cfdcdd.png)