Preferred Height

I have a need right now for a toast to show on a screen that has no status bar and no nav bar. I do however have a bar type element that takes up the upper 66 pixels of the view. There could be a lot of times I imagine where folks might want to have an arbitrary height for their toast. By setting `kCRToastNotificationTypeKey` to `CRToastTypeCustom` and proving some height for the key `kCRToastNotificationPreferredHeight` users can now show a notification of any height.