According to FB2 XML schema <empty-line /> cannot be placed inside
<p>. Linux FBReader can't display such paragraphs, e.g. any "loose"
lists produced by pandoc prior to this commit. Besides that,
FB2 writer placed <p> inside <p> when writing nested lists,
this commit fixes the bug.
Also this commit removes leading non-breaking space from ordered
lists for consistency with bullet lists.
Definition lists are not affected at all.
This is now the default for pandoc's Markdown.
It allows whitespace between the two parts of a
reference link: e.g.
[a] [b]
[b]: url
This is now forbidden by default.
Closes#2602.