Support "New since last review" functionality

[fuzzycode]

Use-case

Similar to how github shows what has changed since you last looked at a PR when revisiting it, it would be nice if code-review could highlight what has changed since you last looked at the PR.

Functionality

It should be possible to know the time that the user opens the buffer or saves the review for future use. That time could be used to determine what is "new" since last visit. It should at least be possible to find out what commits have been added since that time stamp. Ideally it should highlight what files and if possible what hunks have changes since the user last looked at the PR. If #146 is implemented it should probably also be possible to unmark the items as seen if they have changed since the user last looked at the review.