Methods added to plugin classes which simplify changing programs on plugins.

[Enpostive]

This patch adds these methods to both AudioUnit and VST3Plugin classes:

num_programs: "Return the number of programs supported by this plugin" current_program: "Get and set the index of the current program" get_program_name: "Return the name of the program at the specified index" program_list: "The list of programs reported by the plugin"

This is much easier than creating "program_change" MIDI messages (except that process has to be called with reset=False to prevent the program change being reverted) and enables easy access to more than 128 programs in plugins which don't support bank messages (such as Sylenth1)

[Enpostive]

Just realised that this merge has included a change to the pybind11 URL in it. I needed to make this change so that I could clone the repo correctly as the old URL didn't work for me. I'll leave it in as it doesn't change the actual repo, just changes it from SSH to HTTPS. Let me know if I'm doing anything wrong