Add support for div and span attributes

[Witiko]

Closes #126.

Tasks

1. Port fenced divs and bracketed spans syntax extensions (see also Pandoc documentation) from jgm#36.
   1. Port fenced div and bracketed span readers.
   2. Make fenced div and bracketed span writers produce attribute context renderers.
   3. Enable the nesting of fenced divs (adapt from fix: Another attempt fixing nested fenced divs keeping structure jgm/lunamark#52).
   4. Enable fenced divs with implicit classnames and trailing colons (see Support fenced divs with implicit class names and trailing colons jgm/lunamark#59).
2. Add fenced div and bracketed span attribute context renderers.
3. Add blankBeforeDivFence, fencedDivs, and bracketedSpans Lua options.
4. Load fenced divs and bracketed spans syntax extensions in M.new().
5. Add tests
   1. Port unit tests from jgm#36 and jgm#52
      • Add testfiles/lunamark-markdown/fenced-divs.test
      • Fix the blankBeforeCodeFence option:
        The no-blank-before-code-fence.test test works, but fenced-divs.test fails.
      • Add testfiles/lunamark-markdown/no-fenced-divs.test
      • Add testfiles/lunamark-markdown/bracketed-spans.test
      • Add testfiles/lunamark-markdown/no-bracketed-spans.test
   2. Test the interplay between the fencedDivs and blankBeforeDivFence Lua options.
6. Update CHANGES.md