add a "north is up" option

[Fil]

@vasturiano

Btw, I've noticed one of the disorienting things about the versors globe rotation is that it can easily get turned sideways and upside down. I think maintaining a north-up orientation could improve the interaction. This example https://www.jasondavies.com/maps/zoom/ has a "north up" mode, but that's only for the programmatic jumps, not the rotation drag interaction. Do you think it's even possible to enforce a north-up view, or is it a conflicting restriction with the pan interaction?