Publish coverage reports after each test run

[simianhacker]

We need to start publishing coverage reports after each test run. I believe there is a way to integrate this with Jenkins so you have see a chart that shows the trend. Then we can see the progress we are making on our test coverage.

Related to #5800

[cjcenizal]

I'd love to see two outcomes from this:

1. A targeted coverage report, scoped to the plugin(s) that were affected. The Kibana codebase is so vast that a report covering All The Things has too low a signal:noise ratio when all I want to see is coverage in the context of my PR's changes.
2. The change in coverage between master and my feature branch. If this diff is negative, then we can use that as a red flag that we might have overlooked some tests. It will force us to look more critically at our code and make a conscious decision about the level of coverage.