Make Implementing custom matchers and verbs easier

[mysticfall]

For now, implementations of Verb trait don't share much in common, so complicating implementing custom matchers and verbs.

Ideally, there should be a convenient super structure to delegate common tasks to, and an easy way to register them without having to extend FollowedByNegation by an implicit conversion.

[mysticfall]

Currently supported method to implement a custom matcher:

trait SvgTest extends HaveClass {
  this: TestSuite =>
}

trait HaveClass extends Words with Matchers with Conversions

object HaveClass {

  object HaveClassMatcher extends Matcher[SVGElement, HaveClass, String] {

    override def matches(actual: SVGElement, expected: String): Unit = if (!actual.hasClass(expected))
      throw TestFailureException(s"Expected the elment to have class '$expected' but it didn't.")

    override def notMatches(actual: SVGElement, expected: String): Unit = if (actual.hasClass(expected))
      throw TestFailureException(s"Expected the elment not to have class '$expected' but it did.")
  }

  class HaveClass extends AbstractVerb("have class") {

    def apply[A, E](expectation: Expectation[E])(implicit matcher: Matcher[A, HaveClass, E]) =
      WhatIsExpected(this, expectation)
  }

  trait Words {

    val have_class = new HaveClass
  }

  object Words extends Words

  trait Matchers extends MediumPriorityMatchers {

    implicit val haveClassMatcher = HaveClassMatcher
  }

  trait Conversions {

    implicit class FollowedByNegationEx[A](fbn: FollowedByNegation[A]) {

      def have_class[E](expectation: Expectation[E])(implicit matcher: Matcher[A, HaveClass, E]): Assertation[A, HaveClass, E] =
        fbn.builder.assert(WhatIsExpected(Words.have_class, !expectation))
    }
  }
}