Add 'pdc' to ftdetect extensions
Adding highlighting of gfm code blocks
improving the documentation of pandoc executors
require non-whitespace before closing '$'. Should fix #69.

The Pandoc User Guide states:

> Anything between two $ characters will be treated as TeX math. The
> opening $ must have a character immediately to its right, while the
> closing $ must have a character immediately to its left. Thus, $20,000
> and $30,000 won’t parse as math.

Fixed the syntax matching regex to require a non-whitespace before
the closing '$'.
Makes "=" pay attention to document `textwidth`

Previously pandoc would always format to 72-character columns. This will
pull from `textwidth`, assuming it is declared. If it isn't, we'll just
fall back to pandoc's default (72-character columns).
Merge branch 'pull-request-02'