Add mono audio input to PortAudio shim

The way the PortAudio callback works it is possible to reuse the buffer
used for sharing output audio data between Julia and the C-library.

The output data can be pushed from the shared buffer to the PortAudio's
output buffer, after which the same location in the buffer can be used
for storing the data read from the input buffer.

This does assume equal lengths for the in- and output buffers.
Add mutable streaming node example

This demo requires an input hooked up to the default recording device
and will stream that input to the default output device for 10 seconds,
alternating between a muted and unmuted state every second