Support channel:<channel-name> URIs
Support channel:<channel-name> URIs

For convenience, you can now say

  $ nix-env -f channel:nixos-16.03 -iA hello

instead of

  $ nix-env -f https://nixos.org/channels/nixos-16.03/nixexprs.tar.xz -iA hello

Similarly,

  $ nix-shell -I channel:nixpkgs-unstable -p hello
  $ nix-build channel:nixos-15.09 -A hello

Abstracting over the NixOS/Nixpkgs channels location also allows us to
use a more efficient transport (e.g. Git) in the future.

pikajude/nix,dtzWill/nix,zimbatm/nix,dezgeg/nix,shlevy/nix,rycee/nix,ehmry/nix,dezgeg/nix,shlevy/nix,zimbatm/nix,layus/nix,NixOS/nix,shlevy/nix,layus/nix,vcunat/nix,dtzWill/nix,pikajude/nix,peti/nix,NixOS/nix,vcunat/nix,bennofs/nix,ehmry/nix,ehmry/nix,bennofs/nix,peti/nix,zimbatm/nix,peti/nix,shlevy/nix,dezgeg/nix,vcunat/nix,ysangkok/nix,rycee/nix,dezgeg/nix,dtzWill/nix,layus/nix,bennofs/nix,NixOS/nix,bennofs/nix,dezgeg/nix,ysangkok/nix,dtzWill/nix,zimbatm/nix,dtzWill/nix,layus/nix,pikajude/nix,NixOS/nix,rycee/nix,peti/nix,rycee/nix,layus/nix,ysangkok/nix,shlevy/nix,ehmry/nix,peti/nix,ehmry/nix,ehmry/nix,vcunat/nix,rycee/nix,NixOS/nix