2.21.10: * hotfixes for PHP 8
2.21.7: * add some newlines in output to make it better humanly readable
2.21.6: * add missing 'u' inline element
2.21.5: * nl2p: if the double newline contains spaces, e.g. \n \n, these
          spaces are now ignored
        * fixed the direct concatenation: if the body ends without a newline
          and the extended starts without, there is no margin between
          the paragraphs (was broken)

2.21.4: * changed <a> tags to html5 - can now include every block element
        * added a new function which processes html comments. Now everything
          inside an html comment is ignored

2.21.3: Include figure and figcaption tags.
        Thanks to @stephanbrunker!

