Implement MathJax support

Implement MathJax support

* lisp/org-exp.el (org-export-with-LaTeX-fragments): New default t, which
now means to use MathJax processing for HTML.  Also allow new value
`dvipng' to force the old image processing.
(org-infile-export-plist): Parse for MATHJAX setup line.
* lisp/org-html.el (org-export-html-mathjax-options): New option.
(org-export-html-mathjax-config): New function.
(org-export-html-mathjax-template): New option.
(org-export-html-preprocess): Call the LaTeX snippet processor with an
additional argument to declare special ways of processing.
(org-export-as-html): Bind the dynamical variable
`org-export-have-math'.  Insert the MathJax script template when it is
needed by the document.
* lisp/org.el (org-preview-latex-fragment): Call `org-format-latex' with
the additional processing argument.
(org-export-have-math): New variable, for dynamic scoping.
(org-format-latex): Implement specific ways of processing.  New
function argument for processing type.
(org-org-menu): Remove the entry to configure LaTeX snippet
processing.

MathJax is now the default for displaying math in a browser.