Adding alter hook for modifying items while processing entities.

While using this project I encountered limitations where it's difficult to add or modify properties in feed items generated. This PR adds a hook for modifying items before they get output. A few example are included in the documentation on why you might want use this, e.g. adding information about flags from Flag module, or massaging output by adding/removing encoding.

p.s. There's no issue tracker enabled for this project(?)