stdlib: add apt_get_del package_method

This method allows you to pass either `purge` or `remove`, depending on which you want.
```
"unwanted-package"
 package_policy => "delete",
 package_method => apt_get_del("purge");
```
I would've liked to add this to the `apt_get` method, but couldn't figure out how to set the default as `remove`, if nothing is passed. Any hints? :)