shipout should have option to ignore symlinks when deleting old versions

Sometimes, we want to have special symlinks to versions. For example, if we're deploying a documentation website for an API, we might want to have symlinks to specific versions of said API. If we remove the deployed directory to which these symlink(s) point, they no longer work. We should have some mechanism in shipout to determine whether to keep symlinks, (other than 'current', of course), and, if possible, to create new symlinks via some simple set of variables.

An example configuration:

```
"shipout": {
"production": {
"host": "somewhere.example.com",
"port": 22,
"username": "myuser"<
"baseDirectory": "/var/www/something.cool.com",
"keepSymlinks": true, // keep existing symlinks (if created manually)
"createSymlink": "[package.version]" // tell shipout to create a duplicate symlink to the current deployment with the link named whatever the package version is
}
}
```