Add option to visualise the prior distribution

[ntorresd]

In #96 @yungwai-chan suggested the following:

Also it will be great to have option to visualise the prior distribution, especially if this could be customised later.
We should considering adding some additional visualization functionalities for this end after releasing to CRAN.

What do you think? @ben18785, @zmcucunuba, @sumalibajaj, @ekamau

[jamesmbaazam]

Not sure if you are aware of this but priorsense is a nice new package for prior visualisation and diagnostics.

[ekamau]

I think it's useful to be able to visualise the prior distribution. We do it often, but manually, so probably a nice feature to add.