allow different print heads

[ski7777]

Fixes #1

Still work in progress and needs testing after I fixed some issues with my P900NW.
@ogelpre Can you test this after when I'm ready?

[marauder37]

@ski7777 did you get this working? I tried your branch with a P910, and it seems to just feed blank tape until I cut the power.

I had this issue:

    return b'\x1BiM' + functools.reduce(lambda a, b: a | b, self.settings, 0x00).to_bytes()
                                                     ~~^~~
TypeError: unsupported operand type(s) for |: 'int' and 'Settings'

I changed this to

functools.reduce(lambda a, b: a | b, (s.value for s in self.settings), 0x00).to_bytes()

but I don't know whether that's correct.