Update appendToBody.js

Provides an API for #1266

src/directives/appendToBody.js

@@ -1,4 +1,12 @@
 export default {
+    /**
+     * called when the bound element has been inserted into its parent node
+     * (this only guarantees parent node presence, not necessarily in-document).
+     * @see https://vuejs.org/v2/guide/custom-directive.html#Hook-Functions
+     * @param el The element the directive is bound to. This can be used to directly manipulate the DOM.
+     * @param bindings
+     * @param context
+     */
     inserted (el, bindings, {context}) {
         if (context.appendToBody) {
             const {height, top, left, width} = context.$refs.toggle.getBoundingClientRect();
@@ -9,17 +17,24 @@ export default {
                 left: (scrollX + left) + 'px',
                 top: (scrollY + top + height) + 'px',
             });
-            
+            context.$emit('dropdown:appending', { el })
             document.body.appendChild(el);
         }
     },
 
+    /**
+     * called only once, when the directive is unbound from the element
+     * @param el
+     * @param bindings
+     * @param context
+     */
     unbind (el, bindings, {context}) {
         if (context.appendToBody) {
             if (el.unbindPosition && typeof el.unbindPosition === 'function') {
                 el.unbindPosition();
             }
             if (el.parentNode) {
+                context.$emit('dropdown:removing', { el })
                 el.parentNode.removeChild(el);
             }
         }