Add invert to DigitalIn/DigitalInOut/DigitalOut

This Pull Request allows inverting of a Pin.
This is usefull to keep the logic a Programm small and clean.

e.g 
We have a Stepperdriver and a Configfile (on localfilestorage) which defines the used pins (PinName, PinMode, Invert).
Normally en-pin high means that the driver is enabled but with some drivers this is inverted. Same with direction pin, etc.
So we want to set Invert too true to do this else we need to carry this flag through the whole source code.

Another example would be a switch/button. 
If the user use a button which unpressed is closed(true) we can invert the signal.
So the Programm reads true if the button is pressed and the app logic can stay the same.