support insert link

src/RichTextEditor.js

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

src/WebviewMessageHandler.js

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

src/const.js

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