Support for inverted outputs

The SI5351 supports inverting an output; this is e.g. important if one needs to have the same clock with 0° and 180° phase shift, like for driving a Class D PA.

This is pretty straightforward to implement; just set Bit 4 of the respective control register for the clock (inverted) or clear it (not inverted). 

I will add a pull request in a minute.