Fix #20: Load MathJax as necessary, the recommended way
This is arguably the "correct" way to do it. I don't really like it, because it's an external dependency - not right for all use cases etc. But it looks like MathJax wants to only be loaded from a CDN in a web context.
It's likely it'll stay like this until I (or someone else) actually needs to run it in a context where external dependencies are not allowed, e.g. due to company policy, and does the legwork of figuring it out.
For 0.x line, this is Good Enough(tm).
Closes #20 (closed)