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
Parts of the logic has to be there, because Listing.run() has to open to file and read its contents. Hence, at least the logic at the top of run() is needed. Here, fpath is probably fine, though.
(Completely unrelated: won't there be problems when you want to include .html files? .html is added if it's not there in Listings.listing_path(), but in Listings.gen_tasks(), also .html files get .html appended a second time.)