Combine coverage reports

Hello,

Since https://github.com/mattlewis92/karma-coverage-istanbul-reporter was deprecated for angular applications, I switched to this package and I could not find an alternative to `combineBrowserReports` option.
I would like to combine files and summary reports into one for each browser by I'm not sure if it's possible with this plugin, am I right ?

Thanks.

Start by inspecting how the plugin handles the combineBrowserReports option and how it produces browser coverage files and summary reports. Done means determining whether equivalent support can be added so each browser's files and summaries are combined, with the behavior covered by tests.

Written by the indexing model from the issue text.