jackplay: Take an optional argument that specifies port to auto-connect to.

Does not auto-connect if the port is not specified.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
jackplay: Wait for key press before playing if not auto-connecting.

Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>
jackplay: Now use -w option to wait for key press before playback.

Default behaviour of jackplay is now as before, ie it automatically
connects to system playback ports and starts playing immediately.

Closes: https://github.com/erikd/sndfile-tools/pull/25
Signed-off-by: Erik de Castro Lopo <erikd@mega-nerd.com>