Provide more options for bar plot visualization

[zyteka]

Currently, plot_top_contributors can only plot a single entities' features or contributions. We should offer a few more options when multiple entities are fed in, as chosen by a parameter argument:

1. Sum together the absolute values of all contributions for a feature, to get an overall importance
2. Sum together the positive and negative contributions separately, to get the overall positive and negative contributions
3. [maybe] Have one bar per entity per feature

UPDATE: A better alternative to this may be to introduce a global explainer that separates out positive and negative contributions (which would handle the functionality of 2). 1 is handled by global importance.