Sign back in to last team on relaunch

fix for issue https://github.com/wlaurance/slack-for-linux/issues/4. 

Whenever we get a 'document-start' (https://github.com/rogerwang/node-webkit/wiki/Window#document-start) event we cache the sub-domain in localStorage. If the domain is a plain slack domain we assume a logout. Upon start-up we append the iframe with the cached sub-domain as the source url.




What are your thoughts on using an iframe with placeholder HTML as its `src` (or use `about:blank`) and replacing that when this runs?

```html
<iframe nwdisable nwfaketop frameBorder="0" src="data:text/html,<h1>Waiting for &lt;script&gt; to load</h1>"></iframe>
```