refactor: focus around Render/Compact/Pretty/NewWith functions

This is yet another drastic refactor of public API and concepts.
Hopefully the last one, as I'm now fairly happy with things.

render_example_test.go

@@ -39,10 +39,7 @@ func ExampleRender_json() {
 	buf := &bytes.Buffer{}
 	err := render.Pretty(buf, "json", data)
 	if err != nil {
-		fmt.Printf("err: %s\n", err)
-
-		return
-		// panic(err)
+		panic(err)
 	}
 	fmt.Println(buf.String())
 
@@ -142,13 +139,13 @@ func ExampleRender_xml() {
 		Tags: []string{"golang", "json", "yaml", "toml"},
 	}
 
-	// Create a new renderer that supports XML in addition to default JSON, YAML
-	// and Text.
-	r, _ := render.NewPretty("json", "text", "xml", "yaml")
+	// Create a new renderer that supports XML in addition to the default JSON,
+	// Text, and YAML formats.
+	renderer := render.NewWith("json", "text", "xml", "yaml")
 
 	// Render the object to XML.
 	buf := &bytes.Buffer{}
-	err := r.Render(buf, "xml", data)
+	err := renderer.Pretty(buf, "xml", data)
 	if err != nil {
 		panic(err)
 	}