ability to generate the HTML data and .last_run.json files only

[cppgent0]

This is an enhancement request, not a bug/issue

I currently have different types of test runs (eg. unit tests and automated system tests).

Note these are separate processes (not run in parallel).

It would be ideal if I could:

• run each of these separately
• gather the contents of .last_run.json and save if off separately
• have no report data printed. (see output_message in simplecov-html)

And then at the end of all the runs:

• run a script to generate all of the HTML
• generate a report of all the totals from the contents I've gathered from .last_run.json