Add "extension-css" option

[GuillaumeGomez]

It is the continuation of the merged extend-css into rustdoc.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

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[GuillaumeGomez]

Any news?

[alexcrichton]

We generally don't add a configuration option for all rustc/rustdoc options, especially unstable ones. I think it's premature to add an entire Cargo.toml configuration option for this, and the best way to enable this functionality would probably be through RUSTDOCFLAGS similarly to how RUSTFLAGS are handled.

[GuillaumeGomez]

We can keep it open until it's in release version. However, wouldn't it shadow the feature to force people to use cargo flags instead of an option into Cargo.toml?

[alexcrichton]

This will likely require something along the lines of an RFC for dealing with rustdoc options in general in Cargo. There are many more options that want to be specified beyond just this one (e.g. #331).

[alexcrichton]

oops

[alexcrichton]

Ah I've since created a PR for RUSTDOCFLAGS at #2794 so I'm going to close this in favor of that, thanks for the PR though!

[GuillaumeGomez]

Great! Your solution is indeed better. Thanks @alexcrichton!