Listings folder

Hi,

here's my code for implementing the new config variable LISTINGS_FOLDERS from Issue #1499.

It currently contains an ugly hack in plugins/compile/rest/listing.py to fix a bug and implement support for sites using the listing directive without specifying the file's path. I'm currently not sure how to implement that better.

Best,
Felix
Let’s rephrase my request:

```py
def listing_path(self, name, lang):
    name += '.html'
    path_parts = […]
```

Shouldn’t break.