Allow the plugin to manage array & null container

Hi again !

I added few lines to allow the plugin to manage null & array container.

```js
// Null container
document.socialBookmarks(document.getElementById('notPresentInThisPage'));
// Array container
document.socialBookmarks(document.getElementsByTagName('nav'));
```
I really like your library because it is simple, so I understand if it's not in the scope.

I don't apply the patch on the craft version, because I don't know this tool.

![Nothing else](http://media.giphy.com/media/86Ru4rNYwgnok/original.gif)

Cheers,
Thomas.