Customizable nix installer

[FriedrichAltheide]

calamares-nixos-extensions does not currently allow you to easily create a custom installer with a customized configuration.nix. This PR adds the idea of snippets (#15), where many of the previously defined variables containing config parts are moved into individual snippet files. All default snippets are in the defaultConfigs/snippets directory. If you want to create a custom installer, simply place a copy of the snippet - with your changes - in the new `customConfigs/snippets' folder. It will then be used instead of the ones defined in this repository.

As well as snippets, modules can be defined by copying them into the customConfigs/modules folder. Note that if the module is also to be imported, the imports.snippet must be customised using the method above.