Add a disable highlighting option

[SCsupercraft]

It is nice that the highlighting goes away when reloading the page, but sometimes it would still be nice to have a config option to disable the highlighting altogether.

[bmaupin]

Yes, please. I think the highlighting is pretty bad for the reasons already listed in #28

It looks like it's using mark.js

I found a workaround; add this to src/css/custom.css:

mark {
  background-color: unset;
  color: unset;
}