Org reader: Support more inline/display math variants

Support all of the following variants as valid ways to define inline or
display math inlines:

  - `\[..\]` (display)
  - `$$..$$` (display)
  - `\(..\)` (inline)
  - `$..$`   (inline)

This closes #1223.  Again.
Org reader: Improve code by following HLint recommendations

HLint's recommendations for better code are applied to the Org-mode
reader code.
Merge pull request #1229 from tarleb/org-math-improved

Org reader: Support more inline/display math variants