Add combine --keep

[elarivie]

Related to #1108

This pull request adds the CLI option coverage combine --keep

When --keep is present, it does not delete the combined coverage files

[elarivie]

Hello @nedbat,

I see that my pull request if failing some checks...
Is there a way to locally run the checks to ease fixing them?

[nedbat]

Thanks for this pull request! Sure, you can run the tests locally. See https://coverage.readthedocs.io/en/coverage-5.4/contributing.html and let me know if there's something missing or wrong on that page.

Thanks for updating the man page. Don't forget the .rst docs also.

[nedbat]

Thanks! I'll do a light cleanup and add the changelog entry, and put your name in the contributors file.

[nedbat]

This is now released as part of coverage 5.5.