Add documentation for `env_proxy` configuration
Add `Config::Key` helper class
Add `Config::KeyMixin` module
Modify `Config::AptProxy` to use KeyMixin
Add EnvProxy configuration
Add EnvProxy capability for Linux guests
Add Action for configuring EnvProxy
DRY up action hook registering
Merge pull request #6 from tmatilai/env-proxy

Add support for global `*_proxy` environment variables