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
Updated GHC versions in tested-with field.
MediaWiki reader:  Handle table rows containing just an HTML comment.

Closes #1230.
Org reader: Fix parsing of sub-/superscript expressions

This fixes the org-reader's handling of sub- and superscript
expressions.  Simple expressions (like `2^+10`), expressions in
parentheses (`a_(n+1)`) and nested sexp (like `a_(nested()parens)`) are
now read correctly.
Merge pull request #1231 from tarleb/org-fix-subexpr

Org reader: Fix parsing of sub-/superscript expressions
HTML reader:  Treat processing instructions & declarations as block.

Previously these were treated as inline, and included in paragraph
tags in HTML or DocBook output, which is generally not what is wanted.

Closes #1233.
Beamer template:  Adjust widths correctly for oversized images.

Thanks to Garrick Aden-Buie for the patch.
Beamer template: Consistent styles for figure and table captions

Thanks to aaronwolen.
Org reader: Add support for figures

Support for figures (images with name and caption) is added.
Org writer: Fix output for linebreaks

Hard linebreaks in Org mode are represented by the string "\\" as the
last characters in a line.  Adds this feature to the Org-mode writer.
Org reader: Read linebreaks

Linebreaks are marked by the string `\\` at the end of a line.
Fix editorconfig for test files

Test output files should be exactly as defined with regard to trailing
whitespace.  This keeps editorconfig from breaking those files.
Improve handling of hard line breaks in Docbook writer

 * Use a <literallayout> for the entire paragraph, not just for the
   newline character
 * Don't let LineBreaks inside footnotes influence the enclosing
   paragraph
Add some unit tests for Writers.Docbook

These are primarily aimed at testing the new treatment of line breaks,
but hopefully other tests can be added more easily now as features
and changes are implemented in the writer.

Adapted from Tests.Writers.HTML.tests.
Org reader: Removed ANN pragma.

This relies on Template Haskell, which causes problems in Windows
due to libraries with C dependencies.  We need to avoid using TH
in pandoc code.
Merge pull request #1238 from tarleb/org-figures

Org reader: Add support for figures
Merge pull request #1239 from tarleb/org-linebreak

Org linebreaks
Merge pull request #1240 from neilmayhew/master

Docbook output of Line Blocks