Spek integration

[jcornaz]

For specification style:

object ExampleSpec : Spek({
  describe("plus operator") {
    property("is associative") { x, y -> x + y == y + x }
  }
})

And for gherkin style:

object ExampleSpec : Spek({
  Feature("plus operator") {
    property("is associative") { x, y -> x + y == y + x }
  }
})

Note: It would be in a dedicated gradle module. Kwik's users should to opt-in for the Spek extensions. The module name could be something like kwik-spek-jvm (Due to limitations on Spek's side, it cannot be multiplatform at the moment.)