Add two-digit years to permalinks

This adds a :short_year url place holder that allows two-digit years, i.e. years without the century, to be used in urls. It also adds a corresponding entry in site/docs/permalinks.md.

For example:
the permalink: /:year/:month/:day/:title/ -> /2009/04/29/slap-chop/
could become /:short_year/:month/:day/:title/ -> /09/04/29/slap-chop/

the permalink: /:month-:day-:year/:title.html -> /04-29-2009/slap-chop.html
could become /:month-:day-:short_year/:title.html -> /04-29-09/slap-chop.html