Add VISUAL support

[ikalnytskyi]

The 'edit' command uses the 'EDITOR' environment variable to choose the text editor to edit the metadata in. Most software in the wild actually probe 'VISUAL' first before falling back to 'EDITOR'. It'd be nice to behave the same.

See git implementation for reference https://github.com/git/git/blob/c48035d29b4e524aed3a32f0403676f0d9128863/editor.c#L18-L37