Shared subject specs support

[raniejade]

Sometimes it's convenient to reuse specs - like testing a subclass (e.g Calculator and AdvancedCalculator).

DSL

class CalculatorSpec: SubjectSpek<Calculator>({
    ...
});

class AdvancedCalculatorSpec: SubjectSpek<AdvancedCalculator>({
    includeSubjectSpek(CalculatorSpec::class) // or an extension fun itBehavesLike(CalculatorSpec::class)
});

In KSpec we something like this:

class CalculatorSpec: KSpec() {
    override fun spec() {
        describe(Calculator::class) {
            itBehavesLike(calculator())
        }
    }

    companion object {
        fun calculator() = sharedExample<Calculator> {
            describe("add") {
                it("1 + 1 = 2") {
                    assertThat(subject.add(1, 1), equalTo(2))
                }
            }
            ...
        }
    }
}

class AdvancedCalculatorSpec: KSpec() {
    override fun spec() {
        describe(AdvancedCalculator::class) {
            itBehavesLike(CalculatorSpec.calculator())
        }
    }
}

Depends on #87.