Add encoder functionality

I do not believe, currently APM:Rover supports encoders - yet! :open_mouth:

So currently just 2 ESC’s.

That is correct.
Our programmer, Tridge, might want to put it on his ToDo List.
I would like to point out that I do not use encoders on my rover and I won the Sparkfun 2014 AVC Peloton Class in spite of other Peloton Class competitors using encoders.
Encoders are good if you want precision beyond what the present GPS function can provide. However, you must deal with wheel slip and bounce on uneven surfaces that can throw off encoder accuracy.
TCIII Developer