Support configuring hardware monitoring

[sastraxi]

I'd like to be able to configure the 2 hardware headphone monitors on my device (last 2 output channels). I can certainly output sound to them, but I'm unable to configure monitoring as this API isn't exposed.

A quick search shows that kAsioSetInputMonitor of ASIOFuture() needs to be supported and I saw a TODO mention monitoring.

I'd like to contribute by making a pull request, but I don't have any ASIO documentation to work from, just the source mentioned at the end of the file. Any pointers on how I could get started? Thanks!

http://comp.ist.utl.pt/ec-ra/Laboratorio/ASIO%20SDK%202.pdf