parseFeedString allows custom parsing function

I was using your library to read the feed `http://weirdcanada.com/feed`. The feed wasn't parsing (not sure why still), so I wanted to be able to pass in my own function from `String -> Element` for `parseFeedString`.

No behaviour was changed. I added a new method `parseFeedWithCustomparser`. 

PS - if you have any insight into why that url isn't parsing that would be great. I validated the xml and it checked. I think it may be related to the CDATA.