As of 2017-01-23, Github have added this functionality.
You can filter pull requests based their review status (none,
required, approved, changes requested, or required), by reviewer, and
by requested reviewer. For example:
Matches pull requests that have not been reviewed.
Matches pull requests that require a review before they can be merged.
Matches pull requests that a reviewer has approved.
Matches pull requests in which a reviewer has asked for changes.
Matches pull requests reviewed by a particular person.
Matches pull requests a particular person has been asked to review.
A new tab can be seen on personal PR page https://github.com/pulls
A new filter can be seen on the project PR page e.g. https://github.com/symfony/symfony/pulls