Minimum coverage by group

[mak-dunkelziffer]

I think it could be useful to set minimum coverage thresholds per group. Especially when you are trying to get a whole project to 100% test coverage, it makes it easier to complete this "one group at a time" and then "lock the group to 100%", so future devs can't slack off there anymore. Or you could want e.g. your Rails models to have 100% coverage and be less strict with the controllers.

This interface could be possible:

SimpleCov.minimum_coverage_by_group(
  {
    'Models' => { line: 100, branch: 100 },
    'Controllers' => 90 # means primary coverage
  }
)

SimpleCov already has the data for that, because it renders those percentages into the HTML output. Would this be difficult to implement? Would you accept a PR for this?