Add object correlation and on_complete callbacks.

When a user fires off a `Feedzirra::Feed.fetch_and_parse` or a `Feedzirra::Feed.update` it is typical that there is another object that is going to receive the data fetched. By passing in a correlated objects array which corresponds 1:1 with the feed or urls the corresponding objet can be set on the resulting feed object.

In addition, when calling `Feedzirra::Feed.update` it may be desired to be able to hook into an `:on_complete` hook. This pull requests implements both.