Customise MathJax options (allow equation numbering)

[dbeacham]

I'd like to be able to turn on equation numbering in MathJax but that requires editing the config that injected into the haddock page when it's generated.

Not sure of best approach:

1. Support customizing generated HTML headers? #1087 Support customizing generated HTML headers?
2. Allow editing mathjax config
3. Just turn on equation numbering by default. It was turned off by default in MathJax originally to stop change between 1.0 and 2.0.

[harpocrates]

I think either 1 or 2 is fine, with a slight personal preference for 1.

We can also just update the MathJax config options, but it would be nice to understand the impact of doing that first.