Regenerate the cache of command prefixes based on nick etc.

def regenerate_prefixes(self, *args):
        """Regenerate the cache of command prefixes based on nick etc."""
        nick = self.controller.client.user.nick
        self.prefixes = set([
            nick + ": ",
            nick + ", ",
            nick + " - ",
        ])
        # Include lower-case versions as well, but not caps
        self.prefixes.update([p.lower() for p in self.prefixes])
        if self.sigil:
            self.prefixes.add(self.sigil)