add stalebot config (#264)

.github/stale.yml

+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 7
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 14
+# Issues with these labels will never be considered stale
+exemptLabels:
+  - keep fresh
+# Label to use when marking an issue as stale
+staleLabel: stale
+# 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.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: >
+  This issue has been automatically closed for inactivity. If you still wish to
+  make these changes, please open a new pull request or reopen this one.