`predict` method needs to be broken down into multiple functions

[Diego Barneche]

i.e.
predict and posterior_predict
fitted and posterior_epred
The current predict methods need to be rephrased (they're either used for fitted or posterior_epred purposes, really).
Then we can allow other arguments to be passed to the functions via existing brms methods (via @inheritParams roxygen2 tag).
Also need to pull out functionality that automatically creates newdata so the user has the option to 1) use raw data fro predictions; 2) use their own newdata (like many predict methods), or 3) use the package's convenience function to generate newdata.