Add subscribed consumer group count per topic

[weeco]

Use case

Kafka clusters which have grown over time and used by several teams might have topics which aren't used by anyone (anymore). In order to figure out what topics potentially could be deleted it would be helpful to know how many consumer groups consume each topic. An active consumer group for a topic could be defined as a group which has active group offsets for at least one partition for a given topic.

Topics which do not have any consumer groups consuming it are likely good candidates to look at.

[rikimaru0345]

Will be easier to add now that #43 is completed.

[rikimaru0345]

Add column "Consuming Groups" that shows the number of groups that are consuming the topic.

[twmb]

Done?