[Vimprobable-users] Add the :clear command

[Morgan H. <mt...@gm...>]

This patch builds on the prior history setting patch, but I figured it
might be of the most questionable necessity. This adds the ":clear"
command for clearing history. I noticed that the commands processed
from the rc file ended up in the command history, and thought this
should not be the case. That's debatable, but this allows clearing the
history either as the last command in the rc file or manually any
other time.

Morgan