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. We are sharing the expression template, and it has the assumption that there will be multiple legend strings hard-wired in. I set it to 1 here so that whenever the user changes the legend format string it is immediately reflected in the chart. If this were not set, the user would have to change the (hidden) legend dropdown to the first legend string option.