Filter already-installed modules

A few times I've found myself wanting a list of the modules that still need to be installed for a dist (or other perl project).

I made a simple change to `cpanm --showdeps` to filter out already-satisfied preqreqs (https://github.com/miyagawa/cpanminus/pull/207) but miyagawa said he may be deprecating showdeps and he'd like to move more functionality out to separate modules.

Then I remembered this module of yours and thought it might be a nice feature here.

What do you think?