add support for darcs-based feeds

The following patch adds support for darcs-based feeds.
It is tested and works well for my purpose.  I wish it
were committed upstream so that I can share my feed with
other users without converting it to svn/git/whatever.

Best regards,

Signed-off-by: Gabriel Kerneis <kerneis@pps.jussieu.fr>

SVN-Revision: 23616