`stbt.hold` to emulate holding down a button on the remote control

For background see the discussion on the mailing list:
https://groups.google.com/forum/#!topic/stb-tester/fvdWw8o_EjA

I'm not sure if this API for `stbt.hold` is a good idea, given that it
will be quite difficult, if not impossible, to implement for the RedRat
irNetBox unless RedRat implement additional functionality.