Add support for Orion Intelliscope encoders

[synfinatic]

It looks like it would be possible to support Orion Intelliscope encoders. Basically they generate a two channel analog signal which is decoded very much like quadrature decoding for digital encoders, but rather then 'ticks' we have 32 magnetic zones. More info here

Support would require:

1. Supporting analog input for the encoders
2. The encoders generate a .2->4.7V analog signal. So we'd need to convert that to 0->3.3V
3. Write the code to support it.

Obviously item 2 is the biggest issue here. Probably the best solution is an off-board ADC which supports 5V.