Add ability to set custom runner from config

[burbokop]

gradle-scalatest/src/main/groovy/com/github/maiflai/ScalaTestAction.groovy

Line 71 in 94fad79

javaExecHandleBuilder.setMain('org.scalatest.tools.Runner')

Here runner is hardcoded. I think it is properly to give user ablility setting custom runner from build.gradle like:

test {
    runner = 'me.myapp.tools.Runner'
}

And if user not specify runner use default scalatest runner.