Add a plugin for gathering code-coverage from instrumented code.

[adierkens]

As a test-runner, it would be nice to provide the feature (in the form of a plugin) of code-coverage gathered from testing a site that is instrumented with istanbul

The plugin should allow for outputting coverage data in an industry standard format, and the ability to hook into said serialization of the data before writing to disk.

[twinklenotna]

@adierkens can I take this up

[twinklenotna]

@adierkens Can we use codecov.io for this? If yes, I will need some help authorizing codecov.io for this repo.

[twinklenotna]

@adierkens or should we use coveralls?

[adierkens]

@twinklenotna This feature isn't to integrate codecov or coveralls with this repo in particular, but rather to consume a storybook build published using istanbul and produce a coverage report -- similar to what we do for the accessibility plugin.