support "additional targets" in docs.rs metadata

[syphar]

coming from @kpreid on zulip: (#t-docs-rs > changed default targets @ 💬):

It might be helpful if docs.rs supported an additional-targets config, which adds specified targets to the default list. That way, anyone who supports a special target specially for whatever reason (e.g. wasm32-unknown-unknown specific stuff) doesn’t need to also write a copy of the default list that will become stale (e.g. a list written before this Apple target transition, for the sake of including wasm, would now be stale by including x86_64-apple-darwin ).