Support tag option

[st0012]

Sometimes we'll tap multiple objects at the same time and it'll be very useful to add tag to the payload/output messages to help determine the target. for example:

print_calls(seat_assignment_1, tag: "left assignment")
print_calls(seat_assignment_2, tag: "right assignment")

:x [left assignment] # SeatAssignment::GeneratedAttributeMethods
    from: /app/models/seat_picker.rb:228
    <= {}
    => 0

:x [right assignment] # SeatAssignment::GeneratedAttributeMethods
    from: /app/models/seat_picker.rb:230
    <= {}
    => 10

[st0012]

Close via #65