Add experimental::append completion token adapter.

The append completion token adapter can be used to pass additional
completion handler arguments. For example:

  timer.async_wait(
      asio::experimental::append(
        [](std::error_code ec, int i)
        {
          // ...
        },
      42)
    );

  std::future<int> f = timer.async_wait(
      asio::experimental::append(
        asio::use_future,
        42
      )
    );