Feature request: show pretty human readable summary for merged coverage

[huguesb]

I would like an easy way to pretty print for human consumption the contents of merged json coverage files.

Possible solutions that would be satisfying for my use case:

• add an optional flag --display flag to --merge that in addition to writing the output json, also displays the text output on stdout
• add another command (maybe --convert ?) that takes a coverage file produced by slipcover (json, xml, ...) and converts it to another format, crucially supporting conversion to text output

Happy to submit a PR for this feature, but I'd like to make sure I get consensus on the preferred approach before starting the work.