Add a way to group tables into clusters

[tijn]

This could prove more difficult that what you might expect on a first glance. Can a table be part of more than one group? (Should we allow to render the same table twice in the same diagram?)

Would something like this work? Each --group argument defines a new group, it is followed by a comma separated list of tables.

pg-erd --group users,logins,user_groups --group conversations,messages,conversation_partners --group books,book-reviews,book_readings

Maybe --cluster would be a better name for the option. Please leave comments with your opinion on this since I can be quite indecisive about such things.