Adds support for "hidden" property in the manifest.json

Adds support for "hidden" property in the manifest.json. A hidden package will be enabled, but not appear in the Kinds/Modules/Index/Packages lists. This allows you to show only the specific packages you want, but still show inherited kinds/functions from hidden packages that may be subkinded.  

For example, you could mark "enyo" as "hidden":true, and then onyx.Button would still show all inherited functions and allow inline access to enyo.Button and its superkinds, without the added clutter on the left-hand panels.

I realize this is a bit of an odd/specialized/specific change, so I completely understand if you guys decide to not accept the pull request. In which case I'll leave my repo up in the event anyone else needs such a specialization. So no worries either way :)