Make it possible for users to add their own feed filter functions

[karllhughes]

Currently feed items can be filtered by a publishedSince field in the config object. I would like to make this filtering feature more dynamic so that users could include their own filtering functions and rules.

[karllhughes]

This and #10 might be outside the scope of this project.

The goal of this library is to turn RSS feeds into emails; not necessarily manipulate RSS feeds by sorting, filtering, or otherwise mutating them.

These kinds of issues might be better in another project.