Hi,

At Dropbox, we have a desktop client use case which requires cargo-vendor to vendor deps across all platforms (cargo-vendor is great here)

We also have a server use case in which we'd prefer to vendor deps just for linux (currently, we end up pulling in some windows-specific deps, some of which are quite large (eg 50MB of .a files here https://github.com/retep998/winapi-rs/tree/0.3/i686/lib)

Currently cargo-vendor will pull in optional windows dependencies even if we don't need them. I believe this happens in resolve_ws here https://github.com/rust-lang/cargo/blob/master/src/cargo/ops/resolve.rs#L16

It would be great to have an option to exclude windows-specific dependencies for cases where we know we're only building for linux.