lib: add Render::render_{event, prologue, epilogue}

derive push/write automatically from these

examples/jotdown_wasm/src/lib.rs

@@ -7,6 +7,8 @@ use jotdown::Render;
 pub fn jotdown_render(djot: &str) -> String {
     let events = jotdown::Parser::new(djot);
     let mut html = String::new();
-    jotdown::html::Renderer.push(events, &mut html).unwrap();
+    jotdown::html::Renderer::default()
+        .push(events, &mut html)
+        .unwrap();
     html
 }