I came across some `abbr` tags in the wild: check out the first paragraph of http://mindprod.com/jgloss/unmainobfuscation.html. EWW doesn't handle those, so here is a patch for that. My understanding is that there is also an `acronym` tag which has been deprecated in favor of `abbr`. I figured we might as well cover that one too just in case. I picked the `diary` face for abbreviations, but that choice didn't have a lot of thought put into it. Whoever commits can choose something else. Let's try to get this merged ASAP.