Write package coverage data to JSON

[kgryte]

Currently, coverage data is found in HTML files. To ease extraction and metrics, would be good to write coverage data to a JSON file.

The first time coverage data for a particular package is written to this repo, a JSON file should be created and committed. For subsequent coverage runs, coverage data should be appended to the JSON file to allow for tracking coverage over time.

JSON coverage data can then be rolled up/aggregated within namespaces to provide coverage metrics for an entire namespace.

The format of the coverage data can be condensed. E.g.,

[ <num_statements>, <total_statements>, <statements_percentage>, <num_branches>, <total_branches>, <branches, percentage>, ... ]

The condensed format should be explicitly documented.

May also be good to append coverage data to a CSV file, as well, for ease of use in other analysis tools.