title: Possible to incrementally increase coverage with jest?
body: <p>Let's say you already have a repo with zero to little testing coverage. If I were to start setting threshold, I would have to spend the next few weeks just adding tests to increase the coverage.</p>
  
  <p>Instead, is it possible to request that <em>each</em> PR has to always <em>increase</em> the coverage, even if by a ~0.01%? Otherwise, if it's a decrease, then the coverage report should fail, basically saying "Merging this PR will decrease the overall coverage".</p>
  
  <p>Is this possible with <code>jest</code>?</p>