[PR] Add multiple post thumbnails

* [x] If it exists, use the Multiple Post Thumbnails plugin to add thumbnails
* [x] Add Background Image and Thumbnail Image
* [x] Move existing code around images to a new class for better organization
* [x] Add `spine_the_background_image()`, `spine_the_featured_image()`, and `spine_the_thumbnail_image()` to provide a consistent way of displaying post thumbnails.
* [x] Add `spine_has_background_image()`, `spine_has_featured_image()`, and `spine_has_thumbnail_image()` to provide a consistent way of checking for post thumbnails.
* [x] Add `spine_get_background_image_src()`, `spine_get_featured_image_src()`, and `spine_get_thumbnail_image_src()` to retrieve the actual source URL for each post thumbnail.