Add exciting new dimensions to FX by adding mix and amp args:

* mix allows the FX to be fully enabled (1), disabled (0) or to be mixed
  with the original sound (0.5 being half FX half original). Similar to
  the mix arg in reverb
* amp allows the whole output of the FX to be amplified

Not all FX have been converted yet - more to come including modification
of the synth metadata.