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

Actually, this looks pretty neat, but legend strings should be consistent across graphs and pies (and include the metric name). Dunno, maybe go for the standard Prometheus form (`metric_name{foo="bar",biz="buz"}`) for now, as it's used in graphs?