Add ability to filter out headlines that appeared in previous emails

[BrittanyLouviere]

We could look at time of posting to determine this but some sources don't have that on their rss feeds so it may not work for all.

[CharlesLove]

This would require saving a file somewhere. It can be done, but I'd make sure the following are present.

• Last run date (updates with current time at the end of the program)
• (optional) Actual feed articles. This would only be necessary for the rare feeds that don't have dates.