Issue title: Filter When Hook Sent?
 Issue body: Hi Guys!

We're very interested in implementing this to make our Zapier integration snappier!

We have a couple of "brainstorming" question:
Because our system is dealing with phone calls that could be in various states, we only want to trigger events for things with a status of "completed". We wouldn't want the obj.created hook to trigger until the call actually completes.

I'm assuming the only way to do this is to manually fire the obj.created hook when the status of the call changes to "completed". Is that the best way? Is there some way to possibly filter when obj.updated hooks are sent through this API?

Thanks,
Lyle

 Comments: 
 Comment 0: Hey Lyle!

I'd recommend implementing a custom event, if you look at `book.read` on https://github.com/zapier/django-rest-hooks#installing--configuring it should outline a few options for manually calling new events. I can image `call.ringing`, `call.started`, `call.ended`, etc. which would be very cool and powerful!

-bryan

 Comment 1: (Also, I am open to discuss this further! You certainly aren't on your own here.)

 Comment 2: Thanks for the tip! Will update once we start working on this.

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