When unix tools make use of an external editor, they typically check the
environment variable VISUAL and fall back to EDITOR. This commit adds the
additional check for VISUAL to the existing EDITOR check (where VISUAL is
preferred over EDITOR).
Our built-in "diff"-like functionality is pretty good because it's aware of
beets' data structures and types. This makes it more legible, in my opinion,
than an ordinary textual diff. So for now, I'm making this the only option (in
the spirit of making the plugin as straightforward as humanly possible).
