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
`.html.html` makes some sense: `.html.html` for the prettified Nikola-generated source code file, and `.html` for the raw file.