Add experimental image de-duplication using css custom properties

[brettp]

A new flag -x enables an experimental approach to de-duplicating image assets through custom CSS properties. Initial results show significant reduction in sizes:

URL	v2.10.1	PR	Comments
GGGTracker	495K	497K	Seems to not work in either version
Reddit Anime Watch Order	1.1G	187M	Fixes karakeep-app/karakeep#770 (comment)
Gitee SFTPGo Config	1.4G	23M	Problems with JS sidebar nav
Server Part Deals	215M	214M	No significant change
Windcheck	644M	234M	Possibly fixes karakeep-app/karakeep#770 (comment)
Anime PSA – Code Geass	1.1G	188M	Fixes karakeep-app/karakeep#770 (comment)
Win95 Icons	(broken)	2.5M	Also fixes #357

This is significantly under tested. It also seems to add significant browser parsing time, but many of these examples failed to fully load to compare.

Combined with #471, file sizes for sites that use image sprites and named SVG icons will be significantly smaller.