Add versioning for google storage buckets (hashicorp#381)

Author: Stelminator

docs/r/storage_bucket.html.markdown

@@ -58,6 +58,8 @@ to `google_storage_bucket_acl.predefined_acl`.
 
 * `lifecycle_rule` - (Optional) The bucket's [Lifecycle Rules](https://cloud.google.com/storage/docs/lifecycle#configuration) configuration. Multiple blocks of this type are permitted. Structure is documented below.
 
+* `versioning` - (Optional) The bucket's [Versioning](https://cloud.google.com/storage/docs/object-versioning) configuration.
+
 * `website` - (Optional) Configuration if the bucket acts as a website. Structure is documented below.
 
 * `cors` - (Optional) The bucket's [Cross-Origin Resource Sharing (CORS)](https://www.w3.org/TR/cors/) configuration. Multiple blocks of this type are permitted. Structure is documented below.
@@ -86,6 +88,10 @@ The `condition` block supports the following elements, and requires at least one
 
 * `num_newer_versions` - (Optional) Relevant only for versioned objects. The number of newer versions of an object to satisfy this condition.
 
+The `versioning` block supports:
+
+* `enabled` - (Optional) While set to `true`, versioning is fully enabled for this bucket.
+
 The `website` block supports:
 
 * `main_page_suffix` - (Optional) Behaves as the bucket's directory index where