add layer

src/layers.js

@@ -1,5 +1,16 @@
+let id = 0;
 export default class Layers {
   constructor(art) {
     this.art = art;
   }
+
+  add(option) {
+    const { refs } = this.art;
+    id++;
+    refs.$layers.insertAdjacentHTML('beforeend', `
+      <div data-layer-index="${id}" class="art-layer art-layer-${option.name}" style="z-index: ${option.index || id}">
+        ${option.html}
+      </div>
+    `);
+  }
 }