Support Audio Capture in AudioCallbacks

[shybyte]

• Added open_capture method to AudioSubsystem
• Added example audio-capture-and-replay

[shybyte]

@Cobrand This PR aims at solving #625 at least partly.

I know it's not perfect:

• Audio Capture is only implemented for AudioCallbacks but not for open_queue
• It uses the same AudioCallback for capture like for playback which leads to an unnecessary &mut for the case of capture

But it works for my use cases, introduces only minimal changes and does not break the existing API . Considering that #646 might change anyway everything, I hope this PR is useful for now.

[Cobrand]

Looks good to me. I need to test it more thoroughly before merging though, but it should be fine.

[Cobrand]

Wonderful job! Works perfectly on Linux, couldn't test it on Windows but I'm assuming it works correctly in Windows and OSX as well. Could you please confirm which platforms you have tested this on?

[shybyte]

I must admit, that I have tested it only on Linux (Ubuntu 16.04.2 LTS, x86_64, i3 windowsmanager, ALSA).

[Cobrand]

It's supposed to be cross-platform, but even in the case it doesn't work with other platforms, I'm sure someone will raise an issue soon enough.