Improved inline image protocol support

[bptato]

• mlterm and sixel image support is no longer hidden behind undocumented flags, instead they can be selected in the options menu. Note: I left in the -sixel and -ri flags for backward-compatibility, but I'm unsure if they're actually worth keeping since they were undocumented and really should've been config options in the first place. So feel free to remove them, I guess.
• There were two unlink calls that broke image caching and required downloading images with mlterm or img2sixel... which is kind of a disaster IMO as these obviously won't respect w3m proxy settings. So for now I've disabled these unlinks, further investigation is needed to figure out if and when they should be called.
• Support for iTerm2 image protocol is now available; tested with a WIP(?) development build of alacritty which supports sixels and iTerm2 images (and fails very rudely when I try to feed it svg files + won't play gif animations, unfortunately...).
• For iTerm2 I also added a base64_encode function in which unlike encodeB can also encode arbitrary char sequences with null bytes (i.e. images). It should also be somewhat faster than encodeB so I fully replaced that function with this new one.

[tats]

Merged, and updated option setting strings.
Thanks for your contribution.