Make the input and output devices configurable

[dechamps]

Currently, FlexASIO will open the default audio devices as configured in the Windows audio settings, without giving the user a choice. We should add configuration options for manually selecting input and output devices.

The PortAudio device index could be used, but I'm worried that this is unstable when devices are added and removed - best would probably to use a more stable identifier, such as the device name.

For this to be usable, we will probably need to provide a small utility for listing devices.

As a bonus, this option could also be used to disable a stream (input or output), which would be especially useful to avoid glitches due to clock issues, or to make it easier to find a valid format to open a stream (no need to find a format that is valid for both input and output devices).

[dechamps]

The new device option introduced in FlexASIO 0.5, along with the companion program PortAudioDevices, can now be used to customize the device or to disable the input or output entirely.