Fix `\\(...\\)` math rendering in preview.

Reverts 5ed4dbf90315b9e890305d2be3f5532e27b14636.
Added displayMath for self-containment (equal to mathjax default).
Make Pandoc recognize double-backslash delimited math (#46).
Mostly improves latex (and therefore PDF) output which otherwise typesets `\(` etc instead of treating it as math.

Load MathJax in HTML exported from Pandoc.