Re-export some family and formula functions

[athowes]

if we rexport bf and lognormal() etc will 1. make it clear which families we have build support for and which may need new tooling and two mean that users don't need to load or specify brms when using them? What do you think?

Originally posted by @seabbs in #52 (comment)

We could re-export:

1. The families from stats:: and brms:: that we have particular support for (i.e. special prediction functionality) OR all the families from stats:: and brms:: that epidist is compatible with (i.e. all of them)
2. Some functions like brms::bf which are used a lot (any others?)

This would look like (I think):

#' @importFrom brms bf
#' @export
brms::bf

Then run devtools::document().

I'd be in favour of just exporting the families we have prediction functionality for.

[athowes]

Closing as not planned for now.