Ability to execute custom start_link

[stocks29]

https://github.com/arjan/singleton/blob/master/lib/singleton/manager.ex#L58

  defp restart(state) do
    start_result =
      GenServer.start_link(state.mod, state.args, name: {:global, state.name})

    pid =
      case start_result do
        {:ok, pid} ->
          pid

        {:error, {:already_started, pid}} ->
          state.on_conflict && state.on_conflict.()
          pid
      end

    Process.monitor(pid)
    %State{state | pid: pid}
  end

It would be nice if GenServer.start_link/3 here could be replaced by state.mod.start_link/3. That would allow for starting a process which depends on custom code within the start_link/3 function.