add stale configuration (#813)

This PR adds a config for [stale](https://probot.github.io/apps/stale/) to clean up our old issues and PRs.

Author: f2prateek

.github/stale.yml

@@ -0,0 +1,12 @@
+daysUntilStale: 60
+daysUntilClose: 7
+exemptLabels:
+  - pinned
+  - security
+staleLabel: wontfix
+# Comment to post when marking an issue as stale. Set to `false` to disable
+markComment: >
+  This issue has been automatically marked as stale because it has not had
+  recent activity. It will be closed if no further activity occurs. Thank you
+  for your contributions.
+closeComment: true