Add pie charts to promdash

See title. Plz comment with anything I've missed, or additional functionality you would like like.

requested in #114 

@tomcz @juliusv @grobie @ohookins

It is possible, but the easiest way seems to be over-riding this method: https://github.com/PMSI-AlignAlytics/dimple/blob/dd7009ef20785c3c0e840e968bdda0dc672f0ac2/src/methods/_showBarTooltip.js

I could add it in its own folder in `app/assets/javascripts`. Is this something you want to pursue?