ENH add option to do prior predictive checks?

It can hard to set the prior scales properly - it would be nice to be able to do prior predictive checks from within Prophet.

Currently I work with variations of the model in PyMC3 (see [here](https://github.com/pymc-devs/pymc-examples/pull/183) for an example) because then I can easily do prior predictive checks, which help me set my priors sensibly.

I'd be happy to work on a pull request if this was a welcome feature