add threshold-uncovered option
allows threshold to be set by number of uncovered lines instead of percentage of coverage.
In a codebase which continuously grows, if you fix the number of lines uncovered then you (loosely) enforce that all new code should be covered.
closes #168

wow that's nice! 🎉 Any chance that could be merged?
It is super useful when your app has really good coverage, in my case we have:
- 98% coverage
- 20k lines
It means someone can push something with ~200 untyped lines and the coverage won't drop. I could use decimals and get more precise with the required coverage but that PR sounds like a better solution
@rpl Do you think you can take a look at this? This would be nice to get merged.
Bump? :sweat_smile: