feat(RtMidi): quick implementation of timeout for receive

[MrFrangipane]

Hi,

Thank you for this great library.

I'm developping a project that uses MIDI receive in tight loops. In order to quit gracefully, I implemented for RtMidi backend a timeout argument.

I'll have to check deeper into the source of MIDI to complete this pull request.

Is this something useful for you ?

All the best