Support `cargo owner add`

[Manishearth]

$ cargo owner add foo
error: Invalid arguments.

Usage:
    cargo owner [options] [<crate>]
$ cargo owner --help
...
$ cargo owner -a foo

Every. Time.

The error message isn't really helpful there (what's "options"?). We should either fix the error message or just support cargo owner add foo. Both would be good too.

[heisen-li]

The current prompt message is like this:

Do you think it is necessary to provide more detailed help information?Or a better suggestion for modification. @alexcrichton

[ehuss]

I think there are multiple things that can be done here.

cargo owner or cargo owner add currently does nothing. That's a bug in my eyes. I think if you do not pass any of the flags (--add, --remove, or --list), it should be an error.

We could add subcommands instead of using required args. I would probably organize it something like:

cargo owner add OWNER_NAME [CRATE_NAME]
cargo owner remove OWNER_NAME [CRATE_NAME]
cargo owner list [CRATE_NAME]

I'm torn on the idea of inferring the crate name from the current directory, but I guess that should be fine. (Making the user type it explicitly might be a little safer, but is probably not absolutely necessary.)

The old flags should be retained for backwards compatibility.