Change swiper-all-buffer-p to use customize interface

[SterlingHooten]

Swiper comes with some default conditions set to allow for skipping over certain buffers.
In the present implementation these are hard coded in =swiper-all-buffer-p=, so to add an exclusion for buffers (e.g., in pdf-view-mode) it requires either redefining or advising the function.

I propose changing the function to work with the customize group interface (or some other user-definable variable)