Parse `reviewed` event object `state` property returned by Timeline Events API

The Timeline Events API can return a reviewed event object when listing events for a pull request. One of the properties of this object is state, which tells you the state of a submitted review.

The values state can have are:

• commented
• changes_requested
• approved

I'd like to add this field to the Timeline struct defined in issues_timeline.go to make it possible to tell the state of a review when listing pull request events.

[gmlewis]

Fixed by #1886. Closing.