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