A different way to interactively pass options to commands

[Clemens]

Adjusting commands on call is traditionally done via universal prefix
argument. This is mostly nice for numeric repetitions but often it is
also used to change the behavior of a command in other ways and
sometimes even multiple ways depending on the number of times `C-u` is
used.

The recent discussion around changing `interactive' made me think
about ways to improve that. Magit has shown a real nice UI approach to
pass options with its transient menus. Maybe a simpler but similar
mechanism could be added which could also be configured via
`interactive'. Any opinions on this?