Create a GFM (Github Flavored Markdown) parser

As a first step, it supports "fenced code blocks" delimited by three or more
backticks.
Add a test case for GFM parser with CodeRay highlighting disabled
Implement GFM line breaks in paragraphs
Change absolute paths to relative paths in test exclusion list.
Updated GFM parser implementation

* moved test cases to separate directory
* updated parser implementation to avoid code duplication