FEAT: xgboost handler

[isabelizimm]

closes #27

TODO:

• research serialization of xgboost.DMatrix for predict
  ⬆️ Users are able to post a pd.DataFrame for vetiver to do the handling into an xgboost.DMatrix for prediction.

[machow]

The code LGTM w/ Hassan's comments! I wonder if there's a good place to let people find handlers from the docs? (either a written list of which packages are supported, or a list of handlers?)

[isabelizimm]

I wonder if there's a good place to let people find handlers from the docs? (either a written list of which packages are supported, or a list of handlers?)

Good point! I added another tab "Model Types" and updated the front page to reflect what is supported