Nested math within braces

[cben]

MathJax tex input jax is smart enough to parse $y = x^2 \hbox{ when $x > 2$}$. as a single formula. Mathdown parses as 2 (broken) islands.

Depends on #13 — can only be approached sanely if math is recognized by mode.

[cben]

Simply counting braces might be too error-prone during editing: $unmatched { brace$ would not consider the math closed, which is perhaps not an optimal behaviour. Ideally, support nesting only for \mbox and such?

For starterts, supporting nesting with ( / [ delimiters (by only matching the delimiters and not understanding the math structure) would help.