Add option to list only merged pull requests

It looks to me like the [pull request API](http://developer.github.com/v3/pulls/#list-pull-requests) doesn't allow sorting/filtering by date.  One option for listing just merged pull requests would be to use the [issues API](http://developer.github.com/v3/issues/#list-issues-for-a-repository) to get just issues that are pull requests and then check if the pull request [was merged](http://developer.github.com/v3/pulls/#get-if-a-pull-request-has-been-merged).  Though this seems excessively chatty.