Allowing users to specify custom indices to download plugins from would allow people to distribute plugins without having to go through the main krew-index repo. This would be useful at companies where people want to share plugins that are specific to their workflows or aren't appropriate for krew-index.

I wanted to share the proposal doc I had thrown together here https://docs.google.com/document/d/1Im5gmBA6yuKRWgJWbaBjTTFNvkH0me05V3wEeNPU7fE/edit?usp=sharing and opened a new issue for it based on this. There's a lot of similarities to the docs in #23 so if this doesn't add enough I'd be happy to close this.

A lot of ideas came from https://docs.brew.sh/Taps, mainly around how to handle duplicate plugin names.