Add support to initialise opam with multiple repositories

[smorimoto]

I feel a little strange that we can do that when we create a switch, but not when we initialise it. Is there any reason?

[dra27]

When we spend a lot of time working on CI systems, it’s easy to forget that most users ran opam init years ago, and only tend to run it again when they update opam 🙂

[dra27]

However, there’s no particular reason that --repositories couldn’t be available in opam init