text: Title: Improvements to PrecisionRecallCurve
  Body: There are a few things that can be updated in the `PrecisionRecallCurve` visualizer:
  
  - [x] allow the user to specify colors as a list of colors or a colormap and use `resolve_colors` in `_draw_multiclass`. 
  - [ ] refactor shared `_get_y_scores` as a mixin, utility function, or shared base class for `ROCAUC` and `PrecisionRecallCurve`. 
  - [x] Allow user to customize the values of the iso_f1_curves (hard coded to `[0.2,0.4,.06,0.8]` now).
  - [x] Allow user to input training and testing set instead of using  `train_test_split` in the quick method.
  - [x] Remove grid in multiclass case so it's not as busy
  
  See also #602 
  