Fixed max_age setting failing to filter old items

This change fixes #28 (closed) so max_age actually filters older articles by using .total_seconds() instead of .seconds.

timedelta stores the difference between two datetime objects as the number of .days plus the remaining .seconds and .microseconds. This means .seconds is always less than 24 hours worth of seconds (86400). Consequently, .seconds < max_age makes no sense when now - publish_date is longer than one day because the days aren't counted, resulting in false positives with older articles.

.total_seconds() adds up .days, .seconds, and .microseconds as seconds, so .total_seconds() < max_age results in the desired behavior and older articles are ignored.