[markdown] Improve highlighting of lists.

Also includes improved support of nested lists.

Closes #1276.
[markdown] Fix test that was changed in a2ad87f27d021a95f661ac9b106e1f79c6c86a1a.
[markdown] Improve highlighting of blockquotes.

Add support for nesting and avoid hiding code highlighting.

Related to #1276 (but blockquotes, instead of lists).
[markdown] Remove `emstrong` entirely.

Should use `em` or `strong` instead.
[markdown] Only match list items that follow a blank line.

Per Dingus.