Feat: Have ability to return the HTML rather than creating the plots

[zippeurfou]

This is a feature request so not following the template. Let me know if I should anyways.

In tfdv there is the ability to get the HTML using get_statistics_html https://github.com/tensorflow/data-validation/blob/v1.13.0/tensorflow_data_validation/utils/display_util.py#L403-L466. This is pretty handy as sometimes you do want to create HTML files you save on your computer. tfma has a few functions:

• render_slicing_attributions
• render_plot
• render_slicing_metrics
• render_time_series

It would be very handy to be able to just return the HTML for these. For context https://github.com/tensorflow/model-analysis/issues/10 solves render_slicing_metrics but not the others one as far as I am aware (thanks @Bobgy for the help)

[zippeurfou]

Closing this as #10 solves it.