Cache sixel

[lazy-dolphin]

Large images take very long to convert into sixel format. Caching converted sixel would significantly increase image preview speed.

[rabite0]

I used to think caching like ranger does wouldn't be necessary since it's async/doesn't cause stutter and usually it's fast enough, too. But yeah, some things are just going to be slow without caching like previews for compressed tar archives.

I guess it would make most sense to implement support for ranger's caching system. It's simple and flexible enough to work in all those cases and hunter already imports things line bookmarks from ranger, too.

Additionally images could be scaled down before converting them to sixel, as libsixel isn't the fastest library it would probably give a nice boost.