add an option to allow default reviewrs to approve their own pull request

Today, we have a repository with a group of 5 default reviewers.
In the process of creating pull requests, we defined that at list one of the reviewers has to approve the pull request, before it can be merged.
As a result, all the users have to wait for one of the reviewers to approve their pull request before merging.
No one can approve their own pull request, including the default reviewrs.
we would like to maintain the option of approving pull request, yet to add an option to allow default reviewrs to approve their own pull request.