Add RemovableMatchChecker

This checker detects whether match statement in collection methods is removable or not.
[Effective Scala](http://twitter.github.io/effectivescala/) suggests to use pattern matching directly in function definitions whenever applicable.
For example, the checker allows

    list map {
      case Some(x) => x
      case None => default
    }

but does not allow

    list map { item =>
      item match {
        case Some(x) => x
        case None => default
      }
    }

This helps to write pattern matchings clearly.