Suggestion: Add the ability to hold the images into RAM

[mezzelfo]

It would be a great feature the ability to preload all the images into RAM and explicitly hold them once open. I'm not sure if this is already possible but I have not found a way to do it. The way I'm thinking about it, is to have a command line flag arguments such as --force-cache that is disabled by default. If enabled, once an image has been loaded from disk (or from a locally mounted network storage) it would be internally cached in a dictionary.

My use case is that I would like to be able to mount the network storage with external tools and then, passing to feh the local path to the storage, and swiftly switching between images in Slideshow mode. As of today, if the network connection is slow, (VPN over the entire internet), I need to wait feh to refetch the images.

Sorry if this is not a very well thought-out ticket...