Support for multiple channels

[adamcstephens]

Options can differ between channels, so it's helpful to be able to switch views between stable and unstable. It's not clear from the interface which commit is being used to generate the available options.

[SuperSandro2000]

This could already be done partly by using different scopes. We would probably need to implement #244 to be able to hide all then.

The other part I am going to split of into another issue, so that we can display some extra meta information.

[SuperSandro2000]

#281

[SuperSandro2000]

Also see NuschtOS/search.nuschtos.de#73

[MarcelCoding]

I stillt would find it nice, to provide multiple instances for a scope. Scope: Nixpkgs, Intances: 25.05, 25.11, unstable, ...