resolves #492 added math blocks & inline macros, integrated with MathJax
resolves #760 added math style & macro name to use default math engine

- 'math' attribute activates math
- default value of 'math' attribute is 'asciimath'
- use DOMContentLoaded hook to load MathJax
- define constant for math delimiters
change AsciiMath delimiter from ` to \$
Merge pull request #757 from mojavelinux/issue-492

resolves #492 added math blocks & inline macros, integrated with MathJax