text: Title: Endpoint to get Pull Requests for a given repo within a given time frame
  Body: It would be good to have an endpoint in Augur that gets pull requests for a given repo within a given time frame. It would make Augur metrics more uniform and usable for open source health purposes. Similar endpoints to this already exist :
  
  -  [Time series of number of new issues opened during a certain period](https://oss-augur.readthedocs.io/en/main/rest-api/api.html#operation/Issues%20New%20(Repo))
  
  -   [Time series of number of new contributors during a certain period](https://oss-augur.readthedocs.io/en/main/rest-api/api.html#operation/New%20Contributors%20(Repo))
  
  Another api endpoint similar to the ones listed can be created except for Pull Requests instead of Issues.
  