Added  parameter to attachRts that if set will invert the signals:
```
bool attachRts(uint8_t pin, bool invert_signal=false);
```

This should allow more flexability in connecting two teensy 4.x boards to each other and using the CTS/RTS between the two boards for flow control.

This is in response to the thread:
https://forum.pjrc.com/index.php?threads/teensy-4-0-4-1-serial1-attachcts.75899/#post-350107

Where I felt somewhat strange to require somone to have to use hardware inverters to do this.