Allow ad-hoc bypassing passing/failing status check for a specific PR

[fanxia00]

What product do you want to improve?
CodeCov app

Is your feature request related to a problem? Please describe.
There are many cases a low test coverage, either patch or project, is deemed acceptable. For example, refactoring changes cause line of code change which in turn causes coverage change. Whenever this happens, it's cumbersome to deal with - e.g. we'll have to change the yml config and set informational=true or tweak threshold numbers just to change it back once the PR is merged.

#340 (comment) is another category of examples that may benefit from this ad-hoc bypassing feature.

Describe the solution you'd like
Similar to the way GitHub allows skipping checks, users could add a special instruction to their commit message, like:
skip_codecov_check:

Describe alternatives you've considered
fully_covered_patch seems a useful feature, but the 100% patch coverage requirement sometimes is hard to achieve without enough refactoring and the refactoring PRs are usually those getting blocked by the exact problem, which defeats the purpose.