feat(api): add new test runner api

[nicojs]

This adds a new test runner API to Stryker. Differences with the old API:

1. Explicit difference between a dryRun (initial test run) and a mutantRun
   1. A mutantRun result has an activeMutant field indicating which mutant should be activated by the runner
   2. The result types are also explicit. A MutantRunResult now reports back the status as Killed or Survived instead of letting Stryker make that decision.
2. Code coverage interfaces are replaced with the simpler MutantCoverage interface.
3. The MutantRunOptions now has a testFilter, which replaces the old TestFramework API.

I also took the liberty of adding some improvements.

• Add an id field to a TestResult and using that id for the filtering instead of the cumbersome TestSelection interface we introduced to handle string identifiers (names)
• TestResult, DryRunResult and MutantRunResult to be implemented using discrimitated unions. This removes a lot of the optional fields. For example, errorMessage is now a required field on an ErrorDryRunResult.
• Defined errorMessage and failureMessage to be a string, instead of a string[]. If runners want to report more messages, they will have to concatenate themselves.
• A MutantRunResult no longer has to report back all tests that have run. Instead, it only reports the id of the test that killed the mutant (if the status is indeed killed).
• A MutantRunResult no longer has to report back all tests that have run. Instead, it only reports the id of the test that killed the mutant (if the status is indeed killed).
• Added a toMutantRunResult helper function that takes in a dryRunResult and can convert it to a mutantRunResult.

I've added it as TestRunner2, as well as a separate plugin type. This way we can migrate test runner's one by one without breaking the build. Later we can then remove the old test runner API and rename this new one to TestRunner proper.