chore: add Stale to close old pull requests/issues (#98)

Add configuration for the Probot Stale bot, in order to close old
pull requests and issues.

.github/stale.yml

@@ -0,0 +1,12 @@
+# Number of days of inactivity before an issue becomes stale
+daysUntilStale: 14
+# Number of days of inactivity before a stale issue is closed
+daysUntilClose: 5
+# Label to apply when 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 activity occurs in the next 5 days.
+# Comment to post when closing a stale issue. Set to `false` to disable
+closeComment: false