Add --all flag to plugin delete

features/plugin-delete.feature

@@ -12,6 +12,22 @@ Feature: Delete WordPress plugins
       """
     And the return code should be 0
 
+  Scenario: Delete all installed plugins
+    When I run `wp plugin delete --all`
+    Then STDOUT should be:
+      """
+      Deleted 'akismet' plugin.
+      Deleted 'hello' plugin.
+      Success: Deleted 2 of 2 plugins.
+      """
+    And the return code should be 0
+
+    When I run the previous command again
+    Then STDOUT should be:
+      """
+      Success: No plugins deleted.
+      """
+
   Scenario: Attempting to delete a plugin that doesn't exist
     When I try `wp plugin delete edit-flow`
     Then STDOUT should be:

src/Plugin_Command.php

@@ -936,8 +936,11 @@ public function is_active( $args, $assoc_args = array() ) {
 	 *
 	 * ## OPTIONS
 	 *
-	 * <plugin>...
+	 * [<plugin>...]
 	 * : One or more plugins to delete.
+     *
+     * [--all]
+     * : If set, all plugins will be deleted.
 	 *
 	 * ## EXAMPLES
 	 *
@@ -952,7 +955,20 @@ public function is_active( $args, $assoc_args = array() ) {
 	 *     Success: Deleted 1 of 1 plugins.
 	 */
 	public function delete( $args, $assoc_args = array() ) {
+        $all = Utils\get_flag_value( $assoc_args, 'all', false );
+
+        // Check if plugin names of --all is passed.
+        if ( ! ( $args = $this->check_optional_args_and_all( $args, $all, 'delete' ) ) ) {
+            return;
+        }
+
 		$successes = $errors = 0;
+
+        $plugins = $this->fetcher->get_many( $args );
+        if ( count( $plugins ) < count( $args ) ) {
+            $errors = count( $args ) - count( $plugins );
+        }
+
 		foreach ( $this->fetcher->get_many( $args ) as $plugin ) {
 			if ( $this->_delete( $plugin ) ) {
 				WP_CLI::log( "Deleted '{$plugin->name}' plugin." );