Org reader: Precise rules for the recognition of markup

The inline parsers have been rewritten using the org source code as a
reference. This fixes a couple of bugs related to erroneous markup
recognition.
Add .editorconfig

Use EditorConfig to define a some basic styling rules for code.  This
should also help to reduce the number of commits introducing trailing
whitespace.

See http://editorconfig.org/ for details.
Force failure of a Travis build if GHC produces warnings

Pandoc should, as stated in CONTRIBUTING.md, always compile without
warnings.  This patch forces GHC to treat warnings like errors during
Travis builds, making it possible to catch mistakes (like missing
top-level type definitions) early.
Merge pull request #1226 from tarleb/org-emphasis-reader

Org reader: Precise rules for the recognition of markup
Merge pull request #1227 from tarleb/editorconfig

Add .editorconfig
Merge pull request #1228 from tarleb/travis-fail-on-warnings

Force failure of a Travis build if GHC produces warnings
Updated travis script to test with multiple GHC versions.
Fixed $CABAL in travis script.
More travis changes.
More travis tweaks.
Travis: install hsb2hs.
Travis build: remove 6.12; add -v2 to dependencies install.
More travis tweaks.
More travis tweaks.
Travis: gave up trying to test the sdist.

For some reason hsb2hs was not found, even though it was built.
More travis tweaks.
Fixed cabal path in travis config.
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