async renderer.blockcode support?

[suan]

Very similar to this issue on marked, It'd be nice to have async support for using something like pygments in .blockcode. Especially since github-flavored-markdown is so prevalent, and uses pygments for code highlighting.

Also, the pygments example in the README is wrong or outdated, according to the latest pygments code.

[mildsunrise]

:+1:

1. Yeah, but Sundown is, in nature, synchronous. Making it async would involve work in both Robotskirt and Sundown.
2. The Pygments example in the README is indeed ficticious. I just wanted to give a notion of how to use the feature.
3. For now, you can use a synchronous version of Pygments or something like Fibers to accomplish it.
4. Sundown is currently frozen because a new renderer is coming. Better, faster, complete, ...
   Please see vmg/sundown@37728fb2d7137ff7c37d0a474cb827a8d6d846d8

[mildsunrise]

Also, that GFM is prevalent doesn't mean you have to use the same highlighting engine! I'd use a more standard approach like CodeMirror or ACE (they both have the same API and they're JavaScript).