Add label stacking capability

[tylerriccio33]

html( f'{major!s}<span style="font-size: 0.75em; vertical-align: sub; position: relative; line-height: 0.5em;">{minor!s}</span>' )

In that code I stack a major label and a minor label meant for the titling. It would be cool to have something like a GT().stack_labels(...). I make the same argument in #653 .