Merge pull request #3970 from wp-cli/3728-extension-command

Abstract `wp (plugin|theme|theme-mod)` to wp-cli/extension-command

Author: schlessera

composer.json

@@ -49,7 +49,8 @@
 		"wp-cli/server-command": "^1.0",
 		"wp-cli/shell-command": "^1.0",
 		"wp-cli/eval-command": "^1.0",
-		"wp-cli/entity-command": "^1.0"
+		"wp-cli/entity-command": "^1.0",
+		"wp-cli/extension-command": "^1.0"
 	},
 	"require-dev": {
 		"phpunit/phpunit": "3.7.*",