click outside mdl-dialog to dismiss

The "Dismissing dialogs" section of [the material design dialog spec](https://material.io/guidelines/components/dialogs.html#dialogs-behavior) says:

> Dialogs can be dismissed by touching/clicking outside of a dialog

It might be worth adding support for this to mdl-dialog, such as by adding a special CSS class. The mdl javascript could automatically add a click handler that dismisses any currently-visible mdl-dialog with this class when it detects that the user has clicked outside the dialog.

> What MDL Version are you using? (please be specific, e.g. _major.minor.patch_)

1.3.0

Thanks for your consideration and for the great work on mdl!