Math now appears in unicode if possible, without the distracting
italics around identifiers.
Blank lines around headers are more consistent.
Footnotes appear in regular [n] style.
- We no longer include trailing spaces and newlines in the
raw blocks.
- We look for closing tags for elements (but without backtracking).
- Each block-level tag is its own RawBlock; we no longer try to
consolidate them (though `--normalize` will do so).
Closes#1330.
