Add customizable content view with configuration block

This is yet another implementation of an extra view that you can add anything you want to it. It is configured at layoutSubviews, uses a block that is supplied with a pre-sized view, and does not expose previously private variables. I hope this is how you wanted it implemented. If not I can fix it. 

I also added an option for a "sticky toast" where it will stick around until dismissed. On top of that, an activity indicator is shown when the toast is sticky. But that's just unique to the demo.