title: Cargo conditional dependency feature
body: <p>I want to include features of a dependencies dependent on the activation of the feature in my library. If I compile with feature <code>"serde"</code>, I want to install <code>ndarray</code> with <code>"serde"</code> support. And the default should be the default <code>ndarray</code> install.</p>
  
  <p>I' want something like this <code>Cargo.toml</code></p>
  
  <pre><code>
  [features]
  include-serde = ["ndarray-with-serde"]
  
  [dependencies]
  ndarray = { version = "0.x" }
  ndarray-with-serde = { version = "0.x", features=["serde"] }
  </code></pre>
  
  <p>Is this possible at the moment?</p>