Merge pull request #208 from mity/specupd Update to CommonMark spec 0.31 and implement changes it mandates. * Any Unicode of general category P (punctuation) or S (symbol) is now seen as punctuation by the specification. * `<!-->` and `<!--->` are now recognized as HTML comments. * Tag `<source>` has been removed from list of tag trigerring HTML block comment type 4, whereas tag `<search>` added into the list.