Add probot-stale configuration to close stale issues. (#964)

See [probot-stale's](https://github.com/probot/stale) repository for full
list of configuration options.

.github/stale.yml

+# Days without any activity until an issue is labeled as stale
+daysUntilStale: 30
+
+# Days after having the stale label that the issue will be closed
+daysUntilClose: 7
+
+# Labels that prevent issues from being marked as stale
+exemptLabels:
+  - important
+
+# Label to use to identify a stale issue
+staleLabel: stale
+
+# Comment to post when marking an issue as stale. Leave as false
+# to disable.
+markComment: >
+  This issue has been automatically marked as stale because it has not had any
+  activity in the past 30 days. It will be closed in 7 days if no further
+  activity occurs. Thank you for your contributions.
+
+# Comment to post when closing a stale issue. Leave as
+# false to disable.
+closeCommnent: false