Add "keep" label to probot stale excemptions

Author: Dan Hansen

.github/stale.yml

@@ -1,10 +1,13 @@
 ---
+# https://github.com/probot/stale
+#
 # Number of days of inactivity before an issue becomes stale
 daysUntilStale: 30
 # Number of days of inactivity before a stale issue is closed
 daysUntilClose: 7
 # Issues with these labels will never be considered stale
 exemptLabels:
+  - keep
   - pinned
 # Label to use when marking an issue as stale
 staleLabel: wontfix