Add hide_zeros param to confusion matrix

[ExcaliburZero]

Add an optional parameter to the plot_confusion_matrix function to allow the hiding of zero values in the matrix in order to make the plot easier to read in cases where there are many zero values.

Here is what a confusion matrix looks like normally (hide_zeros = False)

Here is the same confusion matrix with zeros hidden (hide_zeros = True)

[reiinakano]

This looks really good!

Though one more thing before I merge. Do you mind adding this to https://github.com/reiinakano/scikit-plot/blob/master/scikitplot/classifiers.py#L56 as well? Thanks!

[ExcaliburZero]

Though one more thing before I merge. Do you mind adding this to https://github.com/reiinakano/scikit-plot/blob/master/scikitplot/classifiers.py#L56 as well? Thanks!

I have now added it there as well.

[reiinakano]

LGTM. Thanks!