Add two-digit years to permalinks

[tmthrgd]

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

[parkr]

I'm down. @mattr-, do you have feelings/thoughts about this?

[mattr-]

feels weird to me but i say “Give the people what they want!!”

[parkr]

Feels weird to me too but mostly because I'll never use it. I agree with the "GIVE THE PEOPLE WHAT THEY WANT!!!" sentiment. #democracy.