Option to use GPIOs on Raspberry Pi (it has no DTR)

[bmidgley]

When wired directly to the raspberry pi uart, there is no way to get a DTR signal. The RTS can ostensibly be brought out using an ALT3 on GPIO17, but that is difficult to configure.

Could we have an override which is just the use of two GPIOs that are wired to GPIO0 and RTS? In my application I could use GPIO27 and GPIO17 for this. Maybe the option could look like

-cg 27,17

As a way to say these "g"pio numbers override DTR and RTS manipulation.

Would you accept a PR that does it like that?