Support for itunes:duration

[diwakergupta]

It doesn't seem possible to access fields like itunes:duration from the FeedItem object. It would be nice to be able to just do something like itemp["itunes:duration"] for properties that don't have explicit getters.

Sample feed here: https://help.apple.com/itc/podcasts_connect/en.lproj/itcbaf351599.html