dev development config

Defined a configuration for running paper-tabs standalone for easier development/contribution.

Small issue... `scrollableTabs` returned from `document.querySelector` doesn't have any functions of the custom element such as `updateBar`. Why is that!?

```js
var scrollableTabs = document.querySelector('#scrollableTabs')
if (scrollableTabs) {
  console.log(scrollableTabs);
  scrollableTabs.updateBar();
} else {
  throw Error("#scrollableTabs element could not be found in the document")
}
```