Logging sinks and exclusions (#178)

* Add sink support to folder module

* Make folder creation optional.

* Add logging sinks to the organization module

* Add logging sink support to project module

* Update readme

modules/organization/outputs.tf

@@ -42,3 +42,10 @@ output "firewall_policy_id" {
     name => google_compute_organization_security_policy.policy[name].id
   }
 }
+
+output "sink_writer_identities" {
+  description = ""
+  value = {
+    for name, sink in google_logging_organization_sink.sink : name => sink.writer_identity
+  }
+}