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.
AFAIK no. It's kind of convoluted that `:proc` of the config option sets the given value to the config. 

However this is not a required thing to get this change in :smile: