add menu clear global config

This pull request is part of https://github.com/yeoman/generator/pull/688

It adds a new menu entry to clear the global config from the command line.

![screen shot 2014-10-31 at 00 04 25](https://cloud.githubusercontent.com/assets/1393946/4853753/1cd030b8-6089-11e4-9ed4-d70dead84ba2.png)
The `existsSync` stub here is needed for the "should not display clear menu" test because the menu entry should only visible if the `.yo-rc-global.json` file is present and contains content.