When an extension is disabled, all of the extensions that depend on this extension should also be disabled.

I've added a function called get_dependent_extensions that returns a list of dependent extension_id's.  Unfortunately, pkg_resources wouldn't give me this list easily, so I had to generate it myself.  I could probably use some fancy list-comprehension to squash that double for-loop, but I figure this one is more readable.

Once again, open to all suggestions and feedback.

Manual - still trying to figure out the best way to test this.