Add child_spec to using macro

lib/websockex.ex

@@ -214,10 +214,32 @@ defmodule WebSockex do
 
   @optional_callbacks format_status: 2
 
-  defmacro __using__(_) do
+  defmacro __using__(opts) do
     quote location: :keep do
       @behaviour WebSockex
 
+      if Kernel.function_exported?(Supervisor, :child_spec, 2) do
+        @doc false
+        def child_spec(conn_info, state) do
+          %{
+            id: __MODULE__,
+            start: {__MODULE__, :start_link, [conn_info, state]}
+          }
+          |> Supervisor.child_spec(unquote(Macro.escape(opts)))
+        end
+
+        @doc false
+        def child_spec(state) do
+          %{
+            id: __MODULE__,
+            start: {__MODULE__, :start_link, [state]}
+          }
+          |> Supervisor.child_spec(unquote(Macro.escape(opts)))
+        end
+
+        defoverridable child_spec: 2, child_spec: 1
+      end
+
       @doc false
       def handle_connect(_conn, state) do
         {:ok, state}

test/websockex_test.exs

@@ -1405,4 +1405,35 @@ defmodule WebSockexTest do
     # A second data tuple means pretty output for `:observer`. Who knew?
     assert {:data, _data} = List.keyfind(rest, :data, 0)
   end
+
+  if Kernel.function_exported?(Supervisor, :child_spec, 2) do
+    describe "child_spec" do
+      test "child_spec/2" do
+        assert %{id: TestClient, start: {TestClient, :start_link, ["url", :state]}} =
+          TestClient.child_spec("url", :state)
+      end
+
+      test "child_spec/1" do
+        assert %{id: TestClient, start: {TestClient, :start_link, [:state]}} =
+          TestClient.child_spec(:state)
+      end
+    end
+
+    test "child_spec is overridable" do
+      defmodule ChildSpecTest do
+        use WebSockex
+
+        def child_spec(_state) do
+          "hippo"
+        end
+
+        def child_spec(_conn, _state) do
+          "llama"
+        end
+      end
+
+      assert ChildSpecTest.child_spec(1, 2) == "llama"
+      assert ChildSpecTest.child_spec(1) == "hippo"
+    end
+  end
 end