pkgdiff-mg: have an option to disable finding LICENSES and COPYING difference

[juippis]

When doing pkgdiff-mg -b against go ebuilds, you can get literally tens of thousands of lines just LICENSE differences due to updated go modules. We can use external tools to verify go licenses.

I'd suggest this to be opt-out, i.e. pkgdiff-mg -b -nl or something alike to ignore licenses.