Stalebot (#1165)

* stalebot: use wontfix-stalebot

I've added two new labels: wontfix-stalebot and pinned.

Wontfix-stalebot will be used by stalebot and pinned can be used to keep
issues open.

* later as well

.github/stale.yml

@@ -6,8 +6,9 @@ daysUntilClose: 7
 exemptLabels:
   - pinned
   - security
+  - later
 # Label to use when marking an issue as stale
-staleLabel: wontfix
+staleLabel: wontfix-stalebot
 # 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