Added support for ``` code blocks

[tbjers]

Added support for GitHub Flavored Markdown code blocks like this:

$ tar cvzf foo.tar.bz2 bar/

[polidore]

hm. this didn't work for me.

[cboettig]

This isn't working for me either. Would be a great feature.

[cboettig]

Looks like the three-tick line which was added in the above commit works for me (when added manually), but that the line is removed and the current script has been adjusted to support n-ticks (as per the designation)?

syntax region mkdCode       start=/\s*``[^`]*/  end=/[^`]*``\s*/

which doesn't work for me. Ideally this would support syntax highlighting by language specified, too.