Allow passing extra arguments to import <nixpkgs>

[SomeoneSerge]

Use-case: running nixpkgs-review with config.cudaSupport = true and custom config.cudaCapabilities.
Currently, nixpkgs-review only tracks affected cuda-enabled packages if they expose a xxxWithCuda attribute in nixpkgs.
Not all packages do that, and it's debatable whether they should.

I think a satisfiable solution would be e.g. being able to run nixpkgs-review pr XXXXXX --extra-nixpkgs-config '{ config.cudaSupport = true; }'. I imagine nixpkgs-review could explicitly state any extra options it has received in the report that it publishes as a github comment, since these options could (and do) affect the eval

[figsoda]

#315

[SomeoneSerge]

Thank you @figsoda!