Pass `--edition` to rustdoc when an edition is configured

[QuietMisdreavus]

In rust-lang/rust#49451 i added an --edition flag to rustdoc that made it compile crates and doctests in the given edition. Cargo should be extended to call this flag when an edition is present in a project's Cargo.toml.

At present the flag is unstable, so a -Z unstable-options will need to be passed to rustdoc for it to work properly. I'm not sure how cargo handles the flag for rustc, but i imagine something similar should happen for rustdoc.

[ehuss]

I can work on this if you'd like.

[matklad]

Thant would be awesome @ehuss !