feat: reporting watermarks

[xdissent]

Passes watermarks from the nyc config to istanbul-lib-report. This should allow custom watermarks for all reporters in istanbul-reports that use them. In the old .istanbul.yml the watermarks lived under a reporting key, so maybe this should be there too, but I read thru some of the other PRs about the new config and thought this might make more sense in the context of those discussions. Plus reportDir was already top level too.

[coveralls]

Changes Unknown when pulling 25b98bd on xdissent:watermarks into ** on istanbuljs:master**.

[bcoe]

@xdissent looking good to me. two asks for you:

1. would it be possible to add a test? an integration test using the bin test suite is great.
2. I think we should document this config setting in https://github.com/istanbuljs/nyc/blob/master/lib/config-util.js#L66

[xdissent]

@bcoe I looked at the tests yesterday and I didn't really see a way to test the watermarks config's effect on anything. It's kinda difficult to add in a custom reporter at the moment, and none of the stock reporters seem to output anything differently based on watermarks - except colors, and colors don't appear to be used in the test output. Any suggestions? For the documentation, I also hit a snag because the watermarks config is an object and yargs isn't very helpful with object args IIRC. Istanbul didn't have cli options for the watermarks either, so I might need some guidance on how that should look if they need to be cli opts now too.

[bcoe]

@xdissent haven't forgotten about this pull, but am digging myself out of a backlog of open-source work; hope to take a look at how one would test this soon 👍

[bcoe]

@xdissent thanks for the contribution, hopefully the first of many. I'm going to try to have a candidate release out today.

[bcoe]

@xdissent would love your help testing:

npm cache clear; npm i nyc@next

If everything looks good in our smoke tests I will promote the release soon. Thanks for the contribution, I look forward to more in the future 👍