Add support for Si4713

[ShadowLight8]

Now that the Transmitter and QN8066 classes are organized, adding an Si4713 class should be much easier. Mostly will be pulling my old code from https://github.com/ShadowLight8/Si4713_FM_RDS and using the new classes.

New Si4713 class
Any specific config options
Anything differ between RPi and BBB?
Document default pins and config

Once tested and merged, update https://github.com/ShadowLight8/Si4713_FM_RDS to inform people of Dynamic_RDS as the long term solution.