Add docs and example of custom reporter implementation

[apsavin]

For wider adoption, gemini should be able to write report in different formats. TAP? JSON? XUNIT? It would be too much work for gemini team to implement all popular formats.

To implement one reporter and document this process is not so hard (still time-consuming, of course, but I think it worth it).

Ref #403

[apsavin]

@arikon asked here why someone prefer webdrivercss. Take a look, for example, on this Just an example, but...