fix #61, again

This fixes an incorrect parsing of backquotes-style code blocks that
start with dashes or equals that could be mistaken for a setext-style
title.
fix: `<pre>` code blocks declared with multiple backquotes

From now on, `<pre>` code blocks declared with multiple backquotes
have to be declared at the beginning of a line, and they have to use
at least 3 backquotes (as for pandoc and github).
fix (#59) paragraph detection in list items and UL vs OL

1. paragraphs were not detected in all cases (notably in ordered lists
when only the last item of the list contained paragraphs)

2. UL and OL detection was inverted (i.e., it should be the first
element of the list that decides, not the last one)