support insert image

src/RichTextEditor.js

     this._sendAction(actions.insertLink, {url, title});
   }
 
+  insertImage(url, alt) {
+    this._sendAction(actions.insertImage, {url, alt});
+  }
+
   setSubscript() {
     this._sendAction(actions.setSubscript);
   }

src/WebviewMessageHandler.js

           zss_editor.prepareInsert();
           zss_editor.insertLink(action.data.url, action.data.title);
           break;
+        case '${actions.insertImage}':
+          zss_editor.prepareInsert();
+          zss_editor.insertImage(action.data.url, action.data.alt);
+          break;
         case '${actions.setSubscript}':
           zss_editor.setSubscript();
           break;

src/const.js

   insertBulletsList: 'INST_BULLETS',
   insertOrderedList: 'INST_ORDERED',
   insertLink: 'INST_LINK',
+  insertImage: 'INST_IMAGE',
   setSubscript: 'SET_SUB',
   setSuperscript: 'SET_SUPER',
   setStrikethrough: 'SET_STRIKETHRU',