Allowing using an external function for default_prior

I and some collaborators are using a modification of BBHPriorDict for a project, and would like to release this modification along with our paper. This small change allows the user to set default-prior to an external function, as is already the case for, e.g., frequency-domain-source-model. I wasn't sure if any modification to the documentation of this feature was really necessary here, or if it was, how best to describe it, so I decided to leave this as part of the review.

Getting this merged isn't extremely urgent--we're still performing some of the runs for the paper--but I thought that it was best to submit it sooner rather than later.