Allow unlimited days from a PHP file

This adds the option to have an all.php file in editions that will be used as a fallback if a N.png/html/php doesn't exist. It can e.g. use $edition_number to have custom output each day.

I haven't added it to the documentation, and an issue is there's then no way of sending a 410 if you have a finite number of things your all.php is outputting - this could be done, though (e.g. have all.php be its own 'type' that calls a function in it instead of just require()ing it).

(Also changed the array to an hash to make the main output section a bit clearer.