Add a manager for prefix modes.

This removes a tiny bit of the ugly logic in IRCParser and hides
it away slightly.

Change-Id: I66b2b17d302186b5e48c839f2fa276f1a2234f6b
Reviewed-on: http://gerrit.dmdirc.com/3953
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager