Override collection url template

Adds the `permalink ` metadata to collections in `_config.yml`. See #2407 (and #2293). The option is used as URL template for documents inside the collection, as opposed to the previously hardcoded value.

I only used currently allowed placeholders (`collection`, `basename` and `output_ext`) but others could be easily added. I wonder whether it would make sense to allow front-matter properties there, but since it would mostly require treatment (like spaces-to-dashes) it may not be as simple as saying it.

Shortnames like `pretty` cannot be used.