GPS passthrough cli command

The commands name is `gpspassthrough`, maybe make it shorter?

Maybe add optional argument for baudrate? (indexed, so 0=115200 etc)

If we should add the argument, how to explain it? Help option? Currently no other command has optional arguments that runs stuff when called without an option, they always seem to display some kind of a list.

Maybe `gpspassthrough -1` (or `*`?) goes on as current baudrate, or it takes an index `gpspassthrough [0-4]`. That way we can have a help text displayed with no arguments?