Cache busting

[nihaals]

I don't know much about the JS ecosystem so I'm not sure if there's already a great tool for this, but it would be nice to have the option of having a hash file name for my assets, including any custom additional assets. This could also probably be extended to also minify the assets. This would all be done during mdbook build and would generate the hashes of all the assets, rename the file (being able to choose between <name>.<short_hash>.<ext> and <long_hash>.<ext> may be useful) and replace any references to the original assets in the generated HTML.