Make users enable to define original ParamType

I want to define original `ParamType`s. as follows

```scala
class ProgrammersController extends SkinnyResource with ApplicationController {
   ...snip

  override def createParams = Params(params + ("hashedPassword" -> params.as[PlainPassword]("plainPassword").hash("dummy salt")))
  override def createForm = validation(createParams,
    paramKey("name") is required & maxLength(64),
    paramKey("favoriteNumber") is required & numeric,
    paramKey("companyId") is numeric,
    paramKey("plainPassword") is required & minLength(8)
  )
  override def createFormStrongParameters = Seq(
    "name" -> ParamType.String,
    "favoriteNumber" -> ParamType.Long,
    "companyId" -> ParamType.Long,
    "hashedPassword" -> ProgrammerParamType.HashedPassword
  )

  private object ProgrammerParamType {
    val HashedPassword = ParamType {
      case v: HashedPassword => v.value
    }
  }

```