Add description of tables support

Author: kyoshidajp

lib/embulk/output/bigquery/bigquery_client.rb

@@ -419,6 +419,7 @@ def create_table_if_not_exists(table, dataset: nil, options: nil)
               table_reference: {
                 table_id: table,
               },
+              description: @task['description'],
               schema: {
                 fields: fields,
               }