Added option for modal-like popups

This adds three new options to enable modal-like behavior:

* `showCloseBtn`: Disables displaying the close button.
* `enableEscapeKey`: Disables dismissing popups by using the keyboard.
* `modal`: Enables modal behavior for a popup, so it's not dismissable by usual means. This is a synonym for setting `closeOnContentClick`, `closeOnBgClick`, `showCloseBtn`, and `enableEscapeKey` to `false`.