add option to not error on "{filename} extra" output

[imsteev]

From mtree(8):

     -e    Do not complain about files that are in the file hierarchy, but not in the specification.

I think this would be useful for when we just want to know if the hierarchy derived from the specification is at least a subset of the actual file hierarchy it is being compared to.

[imsteev]

I agree, this should be implemented after #48 is done. Although TarCheck does check for extra files

[cyphar]

Yes, sorry. I forgot that Extra was implemented (though I'll probably revamp it after #48, which IMO has a nicer interface for differences between manifests).

[vbatts]

This relates to #91