This is a pandoc filter that allows one to use
absolute links but have them be relative to the root directory of your
project. So, a link to /my/path is changed to ../../my/path when
the supplied path to the project root directory is given as ../../.
The path to the root directory must be explicitly specified by the
pathToProjectRoot meta variable (specified in the YAML front matter
or with -M on the commandline).
For an example of it's usage see #1 (comment)