this has the positive side effect that the nixos-generate
binary now also works on configurations
that already import one of the nixos modules
That means, we can use the same cli interface regardless if
the config was pre-produced or is produced by the cli on the fly
fix: #181
* rename eval-format.nix to nixos-generate.nix
Make it clear that it's meant to be used by the nixos-generate script
* rename config.nix to configuration.nix
Bring the name closer to /etc/nixos/configuration.nix
* fix the NIXOS_CONFIG
this is the env name that nixos-rebuild uses
* use -I to load the configuration and format
This has a nice side-effect of allowing URLs as configuration
* add Travis CI
* cleanSource
* gitignore nix result files
* use modulesPath everywhere
this makes sure that the passed pkgs in nixos-generate.nix is the same
one we use to load these other modules
* use toString to avoid evaluation error
