nice versions of prediction / link functions

[mjskay]

• [add|spread|gather]_predicted_samples
  • analogous to rstanarm::posterior_predict, rethinking::sim, brms::predict, modelr::[add|spread|gather]_predictions
• [add|spread|gather]_fitted_samples
  • analogous to rethinking::link, brms::fitted

And the interval versions, which would take a point_interval function (these would just be shortcuts to add_predicted_samples and whatnot + point_interval, but faster by preventing the need to create the larger data frame):

• [add|spread|gather]_predicted_intervals
  • analogous to rstanarm::posterior_predict, rethinking::sim, brms::predict, modelr::[add|spread|gather]_predictions
• [add|spread|gather]_fitted_intervals
  • analogous to rethinking::link, brms::fitted

Might punt the [spread|gather] versions to post-CRAN

[mjskay]

(this is for things like brms, rstanarm, rethinking)

[mjskay]

Base versions are done for CRAN; splitting the rest of this into two post-CRAN issues: #62 and #63