Save the JSON coverage object when --coverdir specified

For some use-cases - related to my day job :) -  I really really want the coverage output of grover to look like something generated out of `istanbul cover` (i.e. `lcov.info`, `lcov-report/` and `coverage.json`).

I added a `json` report format to istanbul and am using it in this PR to generate a JSON file when `coverdir` is specified. Please consider for merge.