[xmonad] new contrib module: X.A.DynamicWorkspaceGroups

[Brent Yorgey]

On my dual-head setup at school, I often find myself using workspaces
in combination to accomplish some task---e.g. having an emacs window
and related things open on one workspace with firefox on the other
workspace to refer to some information.  While working on the task I
want to consider the two workspaces as one "meta-workspace" and be
able to quickly switch to it as a whole without thinking about the
individual workspaces comprising it.  Sometimes I have several of
these "meta-workspaces" at a time.  So I whipped up a simple new
contrib module using extensible state for dynamically creating and
managing such workspace groups.  I've pushed it to the repo, hopefully
others with multi-head setups may find it useful as well.  As always,
patches and suggestions welcome.

-Brent