add quantile_pred

[topepo]

From tidymodels/parsnip#1191

After this PR, we will add a remote to parsnip for this version, then remove the redundant functions.

[simonpcouch]

Before I review, since this is easier for you to affirm verbally than for me to tell via diffs: are there differences in the implementation proposed here vs. the one just merged into parsnip?

[topepo]

The only difference that I can think of is that I had to import as_tibble() to get the S3 dispatch for that to work.

[simonpcouch]

Looking good!