tentatively added support for MathJax and augmented the user manual acco...

...rdingly. I integrated the patch circulating on the mailing list (to which I was pointed by lex)  into xhtml11.conf as well as html5.conf with the notable modification that the 'latexmath' attribute need _not_ be defined in addition to 'mathjax'. another modification is that I've used the https URL for the MathJax site instead of the http URL. it works just fine now as far as I can see: just issue `asciidoc -a mathjax  file.txt' on a file containing some equations in 'latexmath' passthrough blocks (inline as well as display).