Add new category: "bag"

[xsawyerx]

Group packages should allow grouping multiple packages into a single package. This is similar to the CPAN Task:: packages, except they actually bundle all the files together.

"Bundle" might be a better name, except: 1. We already use that word to describe a single package file, and 2. Dist::Zilla's use of "Bundle" (in PluginBundle::) is to reflect the same as Task:: so it doesn't actually reflect the same thing.