Cannot define custom `handle_errors` callback

[TylerPachal]

I have a Phoenix project and in my router.ex file I tried to define a handle_errors/2 callback as defined in the docs, so that I could change some errors I was handing back to users.

Unfortunately this did not work because the handle_errors defined in Plugsnag are being inserted above my definition.

Example:

defmodule ApiWeb.Router do
  use Plug.Router
  use Plugsnag

  # Never gets triggered
  defp handle_errors(conn, %{kind: _kind, reason: _reason, stack: _stack}) do
    send_resp(conn, conn.status, "Something went wrong")
  end
end

I believe that the handle_errors/2 definitions need to be moved from __using__ into a new __before_compile__ block, so that definitions in the application router will be inserted above those in Plugsnag macro.

I can prepare the change if this repo is still active.