Feature Request: Code Coverage

It would be really nice to get coverage reports by using [`nyc`](https://github.com/istanbuljs/nyc):

```
nyc puppet-run plugin:mocha [...mocha options] ./path/to/*.test.js
```

`nyc` automatically instruments the code running in the node process (and any subprocesses), but the bundled code will not be instrumented.

### Potential implementation

I've actually worked on a [similar tool](https://github.com/marko-js/cli/blob/master/packages/test) which also bundles code and runs it in the browser and reports back to the console. We implemented `nyc` support by:

1. [Checking](https://github.com/marko-js/cli/blob/master/packages/test/src/util/browser-tests-runner/bundler.js#L7) if the runner was started by `nyc`
2. Adding an [instrument plugin](https://github.com/marko-js/cli/blob/master/packages/test/src/util/browser-tests-runner/bundler.js#L44-L51) to the bundler
3. Grabbing the [`window.__coverage__` variable](https://github.com/marko-js/cli/blob/72a05034dae808926d89f147af09a73893232dee/packages/test/src/util/browser-tests-runner/browser-dependencies/index.js#L79) which the instrumenting reports to and sending it back to the server

With `puppet-run`, 1 and 3 would remain conceptually the same, but as far as I can tell, Parcel doesn't have an instrumentation plugin. I'm also unsure if it's possible to conditionally apply plugins.

I think the better solution might be to instrument the code after it goes through parcel and rely on sourcemaps to get accurate code coverage. `babel-plugin-istanbul` supports passing an [`inputSourcemap`](https://github.com/istanbuljs/babel-plugin-istanbul#source-maps) so the sourcemaps output by Parcel could be passed along.