rss2email: change -c/--config option from nargs to append.

rss2email: change -c/--config option from nargs to append.

Now you should use

  $ rss2email.py -c path1 -c path2 ... comand

Instead of

  $ rss2email.py -c path1 path2 ... comand

The latter made

  $ rss2email.py -c path1 comand

look like a command-less call (`command' being interpreted as another
config path).