Specify new default values to be used by all subsequent calls to midi_* fns. Merges the specified values with any previous defaults, rather than replacing them
# Sends MIDI :e1 note_on with default opts
# Sends MIDI :e3 note_on to channel 3 on port "foo"
# Sends MIDI :e2 note_on to channel 1 on port "foo".
# This is because the call to use_merged_midi_defaults overrode the
# channel but not the port which got merged in.