value-based-category histograms for clusters

[cornhundred]

Is your feature request related to a problem? Please describe.
We want to be able to see the distribution of value-based-categories in a selected cluster (e.g. hovering over a dendrogram).

Describe the solution you'd like
Show a histogram of the values on mouseover, etc.

Describe alternatives you've considered
Alternatively, we could just show a box plot.

Additional context
This issue was originally raised here MaayanLab/clustergrammer#14, but will be addressed in Clustergrammer-GL first.