Fixed #11: --keep behavior

--keep sets a date in the past so `--keep 1y` means the same date one year before.
Torrents added **before** that date and which are not in catalog are removed.

We were doing the opposite

Author: rgaudin

src/kiwixseeder/runner.py

@@ -161,7 +161,7 @@ def remove_outdated_torrents(self):
         # keep those that are within --keep duration
         keep_until = self.now - datetime.timedelta(seconds=context.keep_for)
         for btih in unselected_books:
-            if self.manager.get(btih).added_on >= keep_until:
+            if self.manager.get(btih).added_on <= keep_until:
                 unselected_books.remove(btih)
 
         if not len(unselected_books):