request: option to also enumerate second-order features

[kentfredric]

A little known thing is you can do:

 cargo test --features "foo/bar"

Which activates the "bar" feature of the "foo" crate. ( it makes sense when you see how features are defined in Cargo.toml ).

Subsequently, this means there is a way to expand test coverage to test situations where somebody turns on that feature in a different crate, and you get it through cargo composition.

So it would be nice if there was a way to test these, understandably, the set size it may generate could be prohibitively large in some cases, so its not a sensible default.

[taiki-e]

Added in 0.4.0 (#70).