Support mutation-score-only reports

[nicojs]

We should support mutation-score-only reports. The JSON looks like this:

{
  "mutationScore": 42
}

The report can look like this:

See stryker-mutator/stryker-handbook#27 for the use case

[nicojs]

Feedback from @hugo-vrijswijk : what will we do with calculateMetrics (implemented in TS and scala). Essentially it would be invalid to call it with a mutation score only report. In fact, the only use case we can think of is the dashboard. Probably better to implement it as an exception there.

Closing this issue for now.