Merge into diff_cover

[DominicOram]

We should try to avoid having to maintain this ourselves and merge it with the wider community. As such we should do Bachmann1234/diff_cover#379

[dperl-dls]

Yes we should, but it'll be a little more involved to add support for all the various things one can specify with arguments and format it to the internal structure of diff_cover.

I'll probably look at this next week.