Added support for new option LISTINGS_FOLDERS.
Fixed ReST listing directive.
Removed flake8 warnings.
Added suggestions by Kwpolska.
Forgot to check in part of code.
Always adding .html. Should fix problem when including .html files, for example via the ReST listing directive.
Fixed syntax error and added more comments.
Moved entries to top.
Added another missing piece.
Added documentation.
Merge branch 'master' of github.com:getnikola/nikola into listings_folder
Fixed missing parameters.
Improved code.
Merge branch 'master' of github.com:getnikola/nikola into listings_folder
Links should now be correct.
Extended FakeSite's config so that tests with ReST compiler won't fail (due to change to its listing plugin).
Making flake8 happy.
Improved config fallback.
Clarify location of `custom.css` file

The current documentation doesn't make it clear that the path of `custom.css` file refers to a path within the `files` directory, so when I tried to create a custom CSS file it wasn't picked up.

Not sure if this change is the clearest way of expressing the correct location but it seemed perhaps better than "you can create a ``assets/css/custom.css`` file in the ``files`` directory and then..."
Further improve wording of custom.css file placement

As per: https://github.com/getnikola/nikola/pull/1512#issuecomment-64617067