Added support for Apple Terminal URL extension.

http://www.macosxautomation.com/lion/terminal.html

    function fish_prompt --description 'Write out the prompt'
      set -l PWD_URL "file://$HOSTNAME"(echo $PWD | sed -e 's/ /%20/g')
      echo -n \e]7";$PWD_URL"\a

      ...
    end