Externalize apt config

This PR introduces the `config` directory and the first application of it is externalizing the configuration of APT into `config/apt/`.
Previously this was hardcoded in the installation script, but now you can adjust it in 'any' way you like.
The `sources.list` file is required, but can be adjusted to include for example 'contrib' and 'non-free'.
Other files ending in `.list` will be added to `/etc/apt/sources.list.d/` and included in this PR is the raspberrypi.org list, which can be deleted if you don't want that :wink: 
You can also included apt preferences file and those files need to end with `.pref` and will be added to `/etc/apt/preferences.d/`. Included in this PR is the preference to install vcgenmod and friends from raspbian and not raspberrypi.org, which was previously hardcoded in the installation script.
Both `.list` and `.pref` files have a special 'variable' named `__RELEASE__` which will be replaced at installation time with the `release` config value.
And then there's the option to include `.key` files in the directory which allows you to add extra repositories key files to APT so that they are trusted.

This would close issue #98