title: Turn off default-features in dependency of dependency
body: <p>I have a chain of dependencies that end in depending <em>optionally</em> on a deprecated library. Concretely, I want to use nalgebra which depends indirectly on rustc-serialize as follows:</p>
  
  <p>nalgebra -> alga -> num-complex -> (optional default) rustc-serialize</p>
  
  <p>I can list the num-complex dependency in my Cargo.toml file and turn off the optional rustc-serialize dependency (<code>num-complex = { version = "0.1.42", default-features = false }</code>), but is there a way to turn off this option all the way up the chain in Cargo.toml?</p>
  
  <p>I have tried an alternative of cloning each of these and manipulating the local copies’ Cargo.toml file to refer to all the local dependencies, which works, but I would like a more maintainable way to do this if possible. </p>