Add pipeline functionality

[jxxcarlson]

Add pipeline functionality so that you can do this:

    type alias Foo =
        { bar : String
        , baz : Int
        }
    fooDecoder : Decoder Foo
    fooDecoder =
       succeed Foo
            |> required "bar" string
            |> required "baz" int

See Yaml.Decode end of file. Has elm-verify-examples tests in the docs for the added required function.