[Proposal] Subcommand to operate on custom registry info

[fa7ca7]

I haven't found this among opened issues. Feel free to close if this idea is already present in some thread.

$ cargo registry -h
Operate on custom registry info

Usage:
cargo registry [options]

Options:
-h, --help              Print this message
-g, --global            Save to $HOME/.cargo/config (else save to .cargo/config)
-a, --add NAME INDEX    Set registry info by name and index URL
-r, --remove NAME       Remove registry info by name
-l, --list              List all the registries (local registries overrides global)

[withoutboats]

I think it would be a good idea to build this out of tree as a custom subcommand first. :-) I think that's how we'd like new subcommands to start out.

[fa7ca7]

Here is my first attempt cargo-reg