How to handle tuning with `glmnet`

[dajmcdon]

To use tune() in {tidymodels}, you have to supply potential values of lambda. The way cv.glmnet() works is to first call glmnet() on the whole data, then do the splits using the sequence from the full fit.

So the proper way to do this with {tidymodels} is simply to mimic that: fit all training data, then pass the path along.

We should incorporate this into the vignette in #105 and #131.