Add option to set max age on RSS import contents

[Ron Kundla]

Some feeds have a really long shelf life and I may move items downloaded to another storage space no longer under torrent control. Those missing feeds will get re-downloaded because the file is missing.

Since I haven't figured out a good way to handle that via Deluge, I thought that adding a max age (in days) would allow us to test the pubDate field and throw away entries older than the max age value relative to the day it was retrieved.

I may take a crack at making the modification myself on a branch, but wanted to float this by the owner in case it was under consideration already.