Allow N, p to be specified in prior

[jeffreypullin]

trafficstars

We could allow the user the specify the value of N and p in the prior.

I haven't already done this because:

• I'm trying to not create extra work to get in the way of the CRAN release
• I think we need to be more confident in our interpretation of what N and p mean

[dvukcevic]

That makes sense. We can an add an interface to this once we finish working through our simulation study.