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
This is part of the feature.  Put it in Features, like this:

```markdown
Features
--------

* New option LISTINGS_FOLDERS
* reST listing directive
* Added Arabic