Allow users to customise MathJax options.

Ref. #1856, #2006, #2045.

This is a more open approach to allow customisation of all MathJax options instead of select options only.

README.md

@@ -1203,6 +1203,8 @@ Reveal.initialize({
 	math: {
 		mathjax: 'https://cdnjs.cloudflare.com/ajax/libs/mathjax/2.7.0/MathJax.js',
 		config: 'TeX-AMS_HTML-full'  // See http://docs.mathjax.org/en/latest/config-files.html
+		// pass other options into `MathJax.Hub.Config()`
+		TeX: { Macros: macros }
 	},
 
 	dependencies: [