code cleanups for commandline.py

Also slight tweaks to a couple of help strings.
cli: add --enableplugin/--disableplugin

Some plugins have been causing trouble for fedup. Allow the user to
manually enable/disable them so they can work around their wonky
plugins.
Complain about broken plugins

If a yum plugin raises an uncaught exception, we should tell the user
which plugin is causing the problem. This way they can disable it with
--disableplugin and they know who to complain to about the problem.