Add support for plugin symbolic links

Useful in when developing a plugin, changes support both adding and uninstalling plugins.
@smistry-toushay You're right, it seems that the referred to version of the shelljs module (`"shelljs": "0.1.x"`) is rather old and doesn't handle symlinks correctly. 

I think that it would be better to upgrade to the latest version instead of working around the issues of the old one.