Add diff command

[noteflakes]

Original idea here:

https://github.com/joram-berger/lydiff

# compare two files
$ lyp diff file1.ly file2.ly

# compare two lilypond versions
$ lyp diff -a2.9.45 -b2.9.46 myfile.ly

The diff command should could provide, in addition to visual diff (using ImageMagic tools), a diff on metadata, such as number of pages, etc, or maybe just a diff of the lilypond stderr output.