Add a page per page metadata to disable MathJax processing

Please consider including the following patch in the next version of pelican-plugins. The use of this patch is to allow disabling mathjax processing on some pages (sometimes, I use dollar-signs in my texts that are not meant to be post-processed, but I also have formula on other scientific pages). This patch is very simple.

PS: first pull-request. 

PPS: Forget this PR, see #321 and #322 instead.