Runner: Allow several pairs of tagging arguments

Using sbt:

```scala
lazy val IntegrationFast = config("it-fast") extend (IntegrationTest)

val testSettings = inConfig(IntegrationFast)(Defaults.testTasks) ++
    Seq(Test, IntegrationTest, IntegrationFast).flatMap {
      scope => Seq(
        testOptions in scope <++= (loadedTestFrameworks in scope, target in scope) map {
          case (fw, t) =>
            fw.get(TestFrameworks.ScalaTest).map {
              _ =>
                val args:Seq[String] = if (scope != IntegrationFast) Seq(
                  "-l", "my.test.utils.tags.FaultyTest"
                )
                else Seq(
                  "-l", "my.test.utils.tags.SlowTest my.test.utils.tags.FaultyTest"
                )
                Tests.Argument(TestFrameworks.ScalaTest, args: _*)
            }.toSeq
        },
        scalacOptions in scope ++= Seq(
          "-language:reflectiveCalls"
        )
      )
    }

```
due to extend relationship "IntegrationFast extend (IntegrationTest)"
I got on it-fast:test
"Runpath must be either zero or two args: List(-l, my.test.utils.tags.SlowTest my.test.utils.tags.FaultyTest, -l, my.test.utils.tags.FaultyTest)"
And it is pretty hard to do a workaround for this.

This pull request should allow using several -l and -n pairs.
In case of `List(-l, my.test.utils.tags.SlowTest my.test.utils.tags.FaultyTest, -l, my.test.utils.tags.FaultyTest)"` 
Will parse to `List(my.test.utils.tags.SlowTest, my.test.utils.tags.FaultyTest)`


This also allows: `-l SlowTests -l PerfTests` and `-n UnitTests -n FastTests` mentioned in example column [Guide](http://www.scalatest.org/user_guide/using_the_runner) and [ScalaDoc](http://doc.scalatest.org/2.0/index.html#org.scalatest.tools.Runner$)