New option in math object to define TeX macros

[JeremyHeleine]

With this small edit, the math object we can define in the options can use the attribute macros. This attribute can be used to define TeX macros that will be available in all the slides. The syntax to use to define macros can be found in MathJax's documentation.

For example we define here a macro without argument and a macro with two arguments:

Reveal.initialize({
	math: {
		macros: {
			R: '\\mathbb{R}',
			set: ['\\left\\{#1 \\; ; \\; #2\\right\\}', 2]
		}
	},

	dependencies: [
		{src: 'plugin/math/math.js', async: true}
	]
});

These macros can then be used in any slide:

<section>
	Here is a common vector space:
	\[L^2(\R) = \set{u : \R \to \R}{\int_\R |u|^2 &lt; +\infty}\]
	used in functional analysis.
</section>