Support MPSSE mode

[Allam76]

Hello. Would it be possible to add support for BITMODE_MPSSE?

[codedstructure]

Yes in theory, though before considering further I'd want specific use-cases (e.g. SPI on FT2232H to device XYZ from pylibftdi running on a Raspberry Pi 4 running raspbian - the sort of level of detail which allows some confidence in testing that it solves someones real problem) from potential users.

My initial concerns would be:

a) does this fit with the existing API, or complicate it much? pylibftdi is intended to be simple / minimal...
b) to what extent does performance matter?