TeX math in org bodies #40

Org bodies show $\dots$, $$\dots$$, (…), […] and \begin{…}…\end{…} as source; the web renders them as MathML. Needs LaTeX fragments in krz/org-swift's parser and a math hook on OrgView, then the same SwiftMath rendering as markdown (#39). Parity row: TeX math as MathML (org half).