Add support for manifest lists

[pranaysashank]

Amazon ecr responds with manifest list irrespective of what we mention in the Accept header. Added an --arch option, that defaults to builtin arch, to fetch manifest & config files for only the required arch.

Strips Authorization Header when following redirects, this is necessary when making requests to AWS as well as curl's default behaviour

I haven't changed the nix code to pass a system option, so it should default to the build machine

Closes #21