Please allow for user defined custom junit runner

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Hello,

right now suites ran with a custom runner (one that is not recognized by pitest) are not recognized and cause 0% coverage and mutation coverage on code tested by them. In my project we simply extended the junitparams.JUnitParamsRunner, but the new runner wasn't recognized by pitest.

I can see that you have a whitelist of custom runners that pitest can work with:

runnerName.equals("junitparams.JUnitParamsRunner")
        || runnerName.startsWith("org.spockframework.runtime.Sputnik")
        || runnerName.startsWith("com.insightfullogic.lambdabehave")
        || runnerName.startsWith("com.googlecode.yatspec")
        || runnerName.startsWith("com.google.gwtmockito.GwtMockitoTestRunner");

Could you please allow for customizing that list, e.g. having a parameter in configuration with names or patterns of allowed runners? Thank you in advance