MIDI effect support for AU

[olilarkin]

If anyone wants to make an iPlug AU Midi controlled effect, you need to make this change to iplugau.r

#if PLUG_IS_INST
kAudioUnitType_MusicDevice,
#elif PLUG_DOES_MIDI
kAudioUnitType_MusicEffect,
#else
kAudioUnitType_Effect,
#endif

[cc_]

Thanks for posting that, it's good to know!