Option to disable fail-fast in sbt testrunner

[hugo-vrijswijk]

The sbt testrunner now reports a mutant as killed as soon as a failing test is found. This is faster and doesn't have any downsides. Unless....

Once we add test information in the reports, the user might want to see all tests that can kill a mutant, instead of the first one that would currently be shown.

https://github.com/stryker-mutator/stryker4s/blob/master/sbt-testrunner/src/main/scala/stryker4s/sbt/testrunner/TestRunner.scala#L47