Allow MIDI messages for all external instrument plugins (#345)

Problem

Some external instrument plugins have audio inputs on the main bus
when loaded in pedalboard. This has been observed with AU and VST3
versions of Native Instruments Reaktor 6 and Arturia ARP 2600 V3, for
example. When attempting to send MIDI inputs to such plugins,
pedalboard throws an error like "ValueError: Plugin '[name]' expects
audio as input, but was provided MIDI messages."

Solution

Instead of using the presence of audio inputs to determine whether
a plugin accepts MIDI messages, use the `isInstrument` value from the
plugin description object.