Add 'git pr' alias for pull-request (#35)

## Summary:
Issue: https://khanacademy.atlassian.net/browse/FEI-3655

## Test plan:
🤞

Author: jaredly

Reviewers: jeresig, lillialexis

Required Reviewers: 

Approved by: jeresig

Checks: 

Pull request URL: #35

.gitconfig.khan

@@ -26,6 +26,7 @@
   db = !git deploy-branch
   rb = !git review-branch
   pb = !git review-branch       ; short for 'phabricator branch'
+  pr = !git pull-request
 
   # Other useful commands.
   outgoing = "!git fetch >/dev/null 2>&1; git log @{upstream}..HEAD"