abbreviations support

[blaenk]

I'm not sure how common this one is, but I first used it in kramdown, Pandoc has support for parsing them but not yet (last I checked) for emitting them, so I used to write my own Pandoc AST transformer to emit them manually, and now that I'm not using Pandoc I do a regex search and replace. I imagine there might be advantages to having this be built into something like pulldown-cmark, in terms of speed, perhaps it enables doing it in the same pass that pulldown-cmark is doing, instead of a separate pass.

[marcusklaas]

This seems like a sensible feature, that shouldn't be too complex to add. There is however the issue of having to do essentially full text search for all the defined abbreviations. If it's hidden behind an option it's probably acceptable, but we should be very explicit to users about the impact on performance.

[ulab]

Just a little heads up: Usually you only need to find the first time an abbreviation is used on a page.

Perhaps the option can be "off, first, all"?