Allow `knife cookbook site share` to omit category

Re-created against master from https://github.com/opscode/chef/pull/2198.

This adds a feature for knife to lookup the category on an existing cookbook. This enables uploading new versions of existing cookbooks with much less pain.

Fixes https://github.com/opscode/supermarket/issues/826 as well.
Ah got it. For some reason Github Search didn't find that in our codebase. Got this. Sorry about the confusion.