Ticket MONGOID-5270 implements tally with a single field arg.

I'd like to propose that tally support multiple field args (as a splat), then return the result as an Array<Hash>. This would allow us to fetch multiple tallies in a single aggregation pipeline pass, saving on time/db resources.

Dog.tally(:color, :size)
  #=> [ { yellow: 9,
  #       brown: 7,
  #       white: 14 },
  #     { small: 10,
  #       medium: 9,
  #       large: 11 } ]