Allow PHP Blueprint Definitions#572

One can create php based blueprints in plugins but currently the autoloaded files from the core folders like site/blueprints only support yaml files.

I would love to create blueprints there with php as well (again, since it was possible in K2).

this is also related to my attepts to create PHP class based blueprints: https://github.com/bnomei/kirby-blueprints

reading either php or yaml is simple enough
$blueprints[$key] = require_once $path;
or in
yaml $blueprints[$key] = Yaml::read($path);

and the issue of having both a php and yaml file with the same name is just a plain user error.

this would also allow a visual bluepprint editor #20 to write/read php files to these folders allowing for better typehinting.