Don't always add a colon to custom commands

The IRC spec mandates that spaces be used as separators between the
arguments for a command. However, the last parameter may itself contain
spaces, and to allow this, one can prefix that argument with a colon.

Instead of checking if the colon is needed, Quassel just always added
a colon to the last argument, which usually works fine. However, it broke
some uses of custom server commands.

This fix now checks if the last argument contains a space or starts with
a colon (which then needs to be escaped), and leaves it alone otherwise.

Fixes #1173 - thanks to Gunnar Beutner for providing the patch.