Merge pull request #3910 from wp-cli/3728-shell-command

Abstract `wp shell` to wp-cli/shell-command

Author: schlessera

composer.json

@@ -38,7 +38,8 @@
 		"wp-cli/export-command": "^1.0",
 		"wp-cli/package-command": "^1.0",
 		"wp-cli/search-replace-command": "^1.0",
-		"wp-cli/server-command": "^1.0"
+		"wp-cli/server-command": "^1.0",
+		"wp-cli/shell-command": "^1.0"
 	},
 	"require-dev": {
 		"phpunit/phpunit": "3.7.*",