Building the router DSL
In order for the use Goldcrest.Router, ... statement to work, we will first need to define a __using__/1 macro in the Goldcrest.Router module. This macro will be responsible for injecting all the router behavior into a module.
Let’s start by simply moving all the plug calls and private parse_body and decode_body_params functions to the __using__/1 macro:
defmodule Goldcrest.Router do   import Plug.Conn   defmacro __using__(opts) do     quote do       use Plug.Router       @opts unquote(opts)       plug Plug.Parsers,         parsers: [:urlencoded, :multipart],         pass: ["text/html", "application/*"]       plug :parse_body       plug :match    ...
