Add ways to get only closed pulls, and only external pulls

Also, why do only two print out when i do
```
$ python show-pulls.py
 4373 edx         carsongee   6  Add Staff Graded Assignments XBlock to edx-platform 1c3f 6+0-  open  18d 20h Jul 10  osc T&L 
 2341 edx            tusbar  16  Add create_account server events to tracking log 1c1f 49+1-  open  57d 11h Jan 28  osc Analytics Doc 
```