Add autoCloseDisabled attribute

The autoCloseDisabled attribute from core-overlay should be exported by paper-toast. If set, toasts do not automatically disappear after interacting with other elements on the page. Toasts would still disappear if they are closed manually or after the proper timeout.

The original and default behaviors of paper-toast are unchanged with this pull request.
Add docs and default value for autoClosedDisabled (see https://github.com/afandria/paper-toast/blob/master/paper-toast.html#L156 as an example)

and you should just do this:
```
autoCloseDisabled="{{autoCloseDisabled}}"
```