Better prefix handling

[AeSix]

There are a few issues with the prefix handling mechanism which have annoyed me slightly over the last week.

One regards prefixes and tab completion, specifically, /b /t /rp, etc to specify where my text will be sent, which if I used these to speak out of my set channel, I cannot tab complete the user names.

When prefixes are hidden, there is no guarantee the prefix will be used, which has left me speaking to my default channel rather than to the channel which the prefix should have sent the text.