Add support for alternate registries

[bjgill]

Currently, we assume that https://crates.io is the only registry. Given that alternate registries have landed/will be landing shortly, we should consider extending support.

[cswindle]

Having a quick look at the code, I think that the things that are required in order to implement this are:

• add a --registry argument (to match the rest of cargo)
• when --registry is provided, cargo-edit should use the cargo library to get the registry index using config->get_registry_index(registry)
• download config.json from the index
• use the API field to use in a renamed fetch_cratesio

Also I would suggest using registry_index_path for crates.io, this way if a user is using another registry (for example a mirror), then they will use the source which is appropriate to their cargo config.

[bjgill]

For reference - alternative registry support has now landed in nightly. See https://boats.gitlab.io/blog/post/2017-10-28-alternative-registries/ for documentation on use.

[cswindle]

An alternative to getting the API using cargo-edit itself, would be to get an interface added to cargo library which allows getting the API (or maybe the whole config.json object from the index), that would then make it a very simple change in cargo-edit.