Support flake modules

[welteki]

Add support for running a vm based on a flake module.

When using the --flake flag, if no attribute is given, nixos-shell tries the following flake output attributes:

• packages.<system>.nixosConfigurations.<vm>
• nixosConfigurations.<vm>
• nixosModules.<vm>

If an attribute name is given, nixos-shell tries the following flake output attributes:

• packages.<system>.nixosConfigurations.<name>
• nixosConfigurations.<name>
• nixosModules.<name>

[Mic92]

I tested it and it looks good to me. Can you also update the readme?

[jackinloadup]

@welteki friendly poke

[welteki]

Apologies for the delay.