Return results for diff and rank methods

[Ayatafoy]

Is your feature request related to a problem? Please describe.
Our use case assumes using wily in the unit tests to validate different metrics thresholds. It is not the best practice to parse cli string to get the metrics, instead, it would be very useful to work with python packages, but without accessing output results in the objects for diff and rank methods it's impossible

Describe the solution you'd like
If we add return results to the diff method and return data to the rank method, we can work with wily in unit tests and operate output in better way

Describe alternatives you've considered
Parsing cli

[devdanzin]

It would be simple enough to add this, but a bit of design choices should be decided before implementation. It could return the whole table as TSV (or JSON if we add that output format), or the lists used to build the table. If we return the lists, it still needs deciding whether to only return the data or headers + data.

What option would better fit your use case?