Added support for pandoc's fenced code blocks

Added support for pandoc's fenced code blocks

Code blocks can be started using three or more ~ characters and stopped
by the same or higher number of ~s. Additionally, pandoc attributes can
be specified. They are ignored in mdp, but may be used to get much nicer
output when converting the md file to html or pdf using pandoc.

For Example:
~~~~ {.bash .linenumbers}
ls $HOME # this is a comment
~~~~

See http://pandoc.org/demo/example9/pandocs-markdown.html