New keyword for merging coverage reports to a single one

[mekxtari]

Is your feature request related to a problem? Please describe.
Currently you can merge coverage reports to a single report in
command line: rfbrowser coverage

It would be nice if i could just just call keyword e.g. Merge Coverage Reports
in Suite Teardown or similar and it would do the trick.

Describe the solution you'd like
New coverage related keyword for merging coverage reports

Describe alternatives you've considered

Additional context

[aaltat]

@allcontributors please add @mekxtari for ideas

[allcontributors]

@aaltat

I've put up a pull request to add @mekxtari! 🎉