Add audio input stream examples

[mitchmindtree]

To begin, it might be nice to port CPAL's record_wav.rs example.

Following this, CPAL's feedback.rs example might also be useful for demonstrating how to run an input and output stream simultaneously and feed audio from one to the other. This might be especially useful seeing as we don't yet have a duplex audio stream API just yet.

[wjakobczyk]

I guess there not much left to be done here, right?