Fast recursive mode for image loading

Add new command line option (-R) for fast recursive image loading.
This new mode doesn't care about the sorting, and instead opens the
first image OS gives through directory read function.

Rest of the images will be then loaded in background, giving instant
startup.

The purpose for this mode is to be used with directories that contains
_lots_ of images for instant viewing.
sxiv manga functions

archive reading through libarchive
dual page mode && manga mode (reverse pages)
manga style panning when in SCALE_ZOOM (1.0)