Report coverage from Node's native --experimental-test-coverage as annotations

[felixfbecker]

Node supports the flag --experimental-test-coverage, which emits coverage to test reporters as the test:coverage event: https://nodejs.org/api/test.html#collecting-code-coverage

It would be awesome if non-covered lines in the PR could be annotated by the reporter with a warning as not being covered.

[MoLow]

Sorry for the delay in the response.
this sounds like a great idea, but I am not sure it should be on by default. we might want to make it configurable, by adding some config file. WDYT?