clear dataset ID when removing controller

closes #155.

inject.js

@@ -227,6 +227,7 @@ chrome.extension.sendMessage({}, function(response) {
               let ctrl = document.querySelector(`div[data-vscid="${id}"]`)
               if (ctrl) {
                 node.classList.remove('vsc-initialized');
+                delete node.dataset['vscid'];
                 ctrl.remove();
               }
             }