Math in text constructions not parsed

[jgm]

E.g.

X^2 = \sum_{ij} \frac{(Y_{ij}-E_{ij})^2}{E_{ij}} \approx \chi^2_{(k-1)(m-1)} \text{under $H_0$}

(from jgm/pandoc#1178)

[jgm]

A simple solution would be to parse this as something like

 [..., EText TextNormal "under ", ESub (EIdentifier "H") (EIdentifier "0")]

Then we don't need to modify the text type, or any of the writers, to allow for math in text.