Addresses Issue #615 : Adding new parameters "Colorbar" and "Heatmap" in PCA visualizer

[percygautam]

The following changes were made to pca.py referenced in #615 :

1. Added new optional parameters 'Colorbar' and 'Heatmap' which shows the magnitude of each feature value to the component, for both '2D' and '3D' cases.

Next Step:

1. Updating Documentation with a proper explanation on how to use these two optional parameters when using PCA visualizer.

P.S: I am adding a snippet showing PCA visualization of the dataset 'credit' from yellowbrick.datasets, when both the parameters are True and proj_dim = 2.

[lwgray]

@percygautam Thanks for the wonderful PR. We are just coming off a hiatus and it might take a bit to get to this but we will asap.

[percygautam]

Sure, no problem.

[percygautam]

@lwgray I noticed that I have not made this PR from feature branch as you have asked in other PRs. Should I close this one and create another one from feature branch?

[lwgray]

@percygautam That would be great if you could create another one from a feature branch.