Enable running specific specs2 examples

[ittaiz]

Enable running specific test "methods"/examples/fragments of specs2 like JUnit and scalatest are supported.
Requires work on both the plugin side and on rules_scala.
@hmemcpy has already done some work on it from both sides

[chaoren]

Only scalatest is left, right?
Still need better filtering options in scalatest, preferably the same as specs2, i.e., "com.example.TestClass#foo should do bar$", so we can reuse code.

[chaoren]

Bug title says specs2. That's finished, so I'll close this.