FR: export the repel workhorse to be available in base plots

[MichaelChirico]

I'm not sure how wrapped up in ggplot the source code is, but it would be great if the workhorse "repellence" algorithm were UI-agnostic so that it could be re-used in base plots

[slowkow]

I would be happy to review a pull request that supports base graphics. This would be a great contribution, because several people have indicated that they would use this feature.