How to config tag editor to support m4a file?

[Angus Zhang]

Hi emms team. appreciate your great work.

I saw that m4a is not listed in `emms-tag-editor-tagfile-functions`. After some
Google search, it seems that is because there is no command line tool for it?

So the best I can do is just write a function to open it with an external GUI
program to edit it maybe? Need confirmation here, don't want to miss anything.

Thanks!