Customise display of date-based sort values

Allows to customise how date are shown next to pages, e.g. from `2014/12/01` to `Mon. 1 Dec 14` — or whatever [date()](http://php.net/manual/en/function.date.php) supports. A blueprint would look like this:

    pages:
       num :
          format: Ymd
          displayformat : D. j M y
          mode: date
          field: created

There is a change in behavior! It defaults to `format` instead of d/m/Y in order to allow to sort pages e.g. only by year, with:

    pages:
       num :
          format: Y
          mode: date
          field: created

(which currently doesn't work)