Support Cargo's "weak dependency features"#56

Reference: - https://rust-lang.github.io/rfcs/3143-cargo-weak-namespaced-features.html - https://github.com/rust-lang/cargo/issues/8832 Example: ```toml [dependencies] serde = { version = "1.0", optional = true, default-features = false } [features] std = ["serde?/std"] ```