Supports the registration of tagged tests in shorthand form in FlatSpec and fixture.FlatSpec.

For example, this class enables syntax such as the following tagged, pending test registration in shorthand form:

"A Stack (when empty)" should "be empty" taggedAs() is (pending)
                                                    ^

In addition, this class indirectly enables syntax such as the following tagged test registration in shorthand form:

"A Stack (when empty)" should "be empty" taggedAs() in { ... }
                                                    ^

Rather than provide in and ignore methods directly, these methods are provided after taggedAs() by implicit conversions because the type passed to in (and ignore) differs in a FlatSpec and a fixture.FlatSpec. A fixture.FlatSpec needs two in methods, one that takes a no-arg test function and another that takes a one-arg test function (a test that takes a Fixture as its parameter). By constrast, a FlatSpec needs only one in method that takes a by-name parameter. As a result, FlatSpec and fixture.FlatSpec each provide an implicit conversion from ResultOfTaggedAsInvocation to a type that provides the appropriate in methods.