🚀 Refine color selection a little more (for xxdiff)

Thanks for making delta! It's amazing!
I'd _really_ love to be able to configure its colors so it looks like the xxdiff color scheme.

See here: https://github.com/blais/xxdiff/blob/master/doc/screenshots/xxdiff-snap-files3.jpg
and here: https://github.com/blais/xxdiff/blob/master/doc/screenshots/xxdiff-snap-files2.jpg

Issues:
- minus and plus lines are used for sections that are modified (yellow in xxdiff). It would be great to be able to independently select the styles of hunks that are changed, over pure insertions and pure deletions.
- non-existing blank lines inserted for alignment aren't stylable right now (I think, tried hard); because I want light colors while using a dark terminal background, those show up inevitably as black.

Please see screenshots.
That would be amazing!

The issue does not name any files, tests, or entry points. Start by locating delta's color configuration and diff-rendering code, then compare how changed hunks and alignment blank lines are represented; done means both cases can receive independent, configurable styles matching the requested xxdiff scheme.

Written by the indexing model from the issue text.