Implement Inverse flag to invert sense of RX and TX like SoftwareSerial

[fourstix]

This pull request implements the inverse flag so that AltSoftSerial inverts the sense of RX and TX similar to the inverse flag in SoftwareSerial library.

Unlike the earlier pull request from @bikemike, the inverse flag affects the sense of both the RX and TX pins. The earlier request only inverted the sense of one line, so it was a partial implementation. This pull request is a full implementation.

This code has been tested with an inverse serial line device, and a device running with the normal sense of the lines. Code that ran fine with AltSoftSerial runs fine with this change.

I implemented this change because I needed to reverse the sense of the lines to communicate with a particular piece of hardware, but I also need full-duplex communication.

This change allows me to run AltSoftSerial to achieve those goals.

[TheGreyGhost]

Thanks dude, I copied your code even though it hasn't been merged (yet).
-TGG