Add hpf, lpf and mono forcer to mixer with pass through options:

* Useful for situations where you perform with Sonic Pi on other people’s
  PA systems which may not support stereo (i.e. typical night clubs) and
  may not care for super low/high frequencies blasting through…
* Will connect these params to an advanced audio preference pane soon.
Add stereo inverter to mixer: 

* for situations where it’s easier to toggle a preference than to swap
  the position of the speakers.
Add initial support for new advanced audio features:

* Invert stereo
* Force mono
* HPF
* LPF

Plugged stereo inversion into GUI - need to plug the rest in and tidy.
Teach fns which accept a path to understand ~/ as the home dir:

i.e. sample “~/Desktop/foo.wav”