Make the input and output channel counts configurable

[dechamps]

Backends behave differently depending on the channel count specified when opening the stream. For example, a backend might decide to downmix when the channel count exceeds the physical channel count of the device.

Currently, FlexASIO opens the device with the maximum device channel count advertised by PortAudio (if we ignore #3). This might not be what the user wants. Besides, I've seen a few cases where the channel count provided by PortAudio doesn't make a ton of sense (e.g. MME advertising 8 channels for a 5.1 device).

For this reason, it would make sense to allow the user to customize the input and output channel counts. I don't think it would be enough to just count the number of channels that the ASIO host application requests in createBuffers(), because that's somewhat ambiguous (there are scenarios where the user might want to use only N channels, but open the device with M > N channels). One exception would be when the requested channel count is zero (see #24).

[dechamps]

The new channels option introduced in FlexASIO 0.5 can now be used to customize the channel count.