support value passed to --with_attachments (fix wp-cli#40)

src/Export_Command.php

@@ -155,6 +155,7 @@ public function __invoke( $_, $assoc_args ) {
 			'with_attachments',
 			$defaults['with_attachments']
 		);
+		$this->export_args['with_attachments'] = preg_match('/^(?:y|yes|1|true)$/i', (string)$this->export_args['with_attachments']); // nb: (string)TRUE === "1"
 
 		if ( !function_exists( 'wp_export' ) ) {
 			self::load_export_api();

src/WP_Export_Query.php

@@ -196,7 +196,7 @@ private function post_type_where() {
 			return;
 		}
 
-		if ( $this->filters['with_attachments'] == FALSE && ( !$this->filters['post_type'] || !in_array( 'attachment', $this->filters['post_type'], TRUE ) ) ) {
+		if ( $this->filters['with_attachments'] && ( !$this->filters['post_type'] || !in_array( 'attachment', $this->filters['post_type'], TRUE ) ) ) {
 			unset( $post_types['attachment'] );
 		}