perf: make the most of streamed rendering

src/emitters/simple-emitter.ts

@@ -2,13 +2,9 @@ import { stringify } from '../util/underscore'
 import { Emitter } from './emitter'
 
 export class SimpleEmitter implements Emitter {
-  public html: any = '';
+  public buffer = '';
 
   public write (html: any) {
-    this.html += stringify(html)
-  }
-
-  public end () {
-    return this.html
+    this.buffer += stringify(html)
   }
 }