add example and test for exporting to stdout

Author: miya0001

features/db-export.feature

@@ -20,7 +20,7 @@ Feature: Export a WordPress database
       """
     And the wp_cli_test.sql file should exist
 
-  Scenario: Exclude tables when exporting the dtabase
+  Scenario: Exclude tables when exporting the database
     Given a WP install
 
     When I run `wp db export wp_cli_test.sql --exclude_tables=wp_users --porcelain`
@@ -33,3 +33,12 @@ Feature: Export a WordPress database
       """
       wp_options
       """
+
+  Scenario: Export database to STDOUT
+    Given a WP install
+
+    When I run `wp db export -`
+    Then STDOUT should contain:
+      """
+      -- MySQL dump
+      """

src/DB_Command.php

@@ -302,6 +302,15 @@ public function query( $args, $assoc_args ) {
 	 *     $ wp db export --exclude_tables=$(wp db tables --all-tables-with-prefix --format=csv)
 	 *     Success: Exported to 'wordpress_dbase-db72bb5.sql'.
 	 *
+	 *     # Export database to STDOUT.
+	 *     $ wp db export -
+	 *     -- MySQL dump 10.13  Distrib 5.7.19, for osx10.12 (x86_64)
+	 *     --
+	 *     -- Host: localhost    Database: wpdev
+	 *     -- ------------------------------------------------------
+	 *     -- Server version	5.7.19
+	 *     ...
+	 *
 	 * @alias dump
 	 */
 	public function export( $args, $assoc_args ) {