Allow deriving Jason.Encoder with sorted keys

[gilbertbw]

I think this closes #170, but rather than making a change to pretty_print, I modified @derive Jasen.Encoder

[michalmuskala]

Sorry for a late reply. I wonder if a simpler approach might be to preserve the order of keys passed to only: ... and document that it will be preserved. That way the user will be in full control over what order the keys are encoded in when deriving.