Add revision history for hook source code

[Marak]

Users should be able to go back in history at least 10 versions to see previous saved versions of the hook's source code and settings.

We are already storing the revision history for all Hook services in CouchDB. We'll need to surface these document revisions in the API and build a simple UX for the /admin page.

We'll also need to be mindful of the CouchDB configuration so that we don't accidentally compact / prune the revision history during database cleanup / compaction. RE: #65

[pyhedgehog]

Isn't it provided by gist already? And choosing between gist and hookio-local includes this drawback.
From my PoV it would be better to implement more hook source providers - like BitBucket's snippets.

[pyhedgehog]

I was wrong - bitbucket's snippets is unusable in this case - alternative to public they has "private" in contrast to "secret" in gist. "Secret" gist can be accessible by anyone who knows url, but "private" snippet can be accessed only by owner.

[Marak]

@pyhedgehog - Gist is not the only way to give hook source code.

Hook source can also be stored directly on hook.io site without gist.

Still need revision history for non-gist sources.

[pyhedgehog]

And choosing between gist and hookio-local includes this drawback.

Gist is not the only way to give hook source code.