Add configuration for Apt proxy

This PR adds support for configuring proxies for Apt. Even with the apt bucked enabled it makes sometimes sense. For example all developers can use a single apt-cacher-ng instance in a corporate network.

If you think this is a good idea I can add some documentation to the readme (although I'm very crappy writing anything =)).
But if you feel this is out of the scope for this plugin no problem. I can make a separate plugin and won't rage quit using this plugin. ;)