If no other reason is given for parting or quitting with
rcirc-cmd-part or rcirc-cmd-quit, rcirc will use the value of
rcirc-id-string: "rcirc on GNU Emacs <version>". Kinda stuffy.

This patch adds two customizable variables to rcirc:
rcirc-default-part-reason and rcirc-default-quit-reason. The above
commands will use these variables as the default part/quit reason if
they are set. If they are left nil, rcirc will fallback to its current
behaviour, and use the value of rcirc-id-string.


Changelog entry:

2012-11-10  Daniel Ralston  <Wubbulous@gmail.com>

    * rcirc.el: Make the default PART and QUIT reasons used by
    rcirc-cmd-part and rcirc-cmd-quit customizable.
    (rcirc-default-part-reason,rcirc-default-quit-reason): New
    customizable vars.