Add a way not to enable features in workspace crates

[jyn514]

This is a direct copy of facebookarchive/cargo-guppy#655; let me know if I should add more details from the discussion there.

I have a workspace that looks like this, where my-crate: feature1 => x/feature2 means "my-crate has a feature named feature1 which enables feature2 in the dependency x":

bin/my-app: fips => transitive-dep/fips: => boring-sys/fips

When I run cargo hakari generate, it adds a line to workspace-hack/Cargo.toml that looks like

transitive-dep = { git = "...", branch = "...", default-features = false, features = ["fips"] }

I think haraki is running cargo tree --all-features or something like that. I want it not to enable that feature; it only builds on linux machines and some of the people on my team use Mac. I don't see an existing way to do this:

$ cargo hakari generate -h
Generate or update the contents of the workspace-hack crate

USAGE:
    cargo-hakari generate [OPTIONS]

OPTIONS:
        --color <COLOR>    Produce color output [default: auto] [possible values: auto, always,
                           never]
        --diff             Print a diff of contents instead of writing them out. Can be combined
                           with `--quiet`
    -h, --help             Print help information
    -q, --quiet            Suppress output
    -v, --verbose          Produce extra output

Would it be possible to add one?