title: missing Cats Functor[Future] instance
body: <p>I am trying to use <code>OptionT</code> to combine methods returning <code>Future[Option[T]]</code> in a for-comprehension. </p>
  
  <pre><code>import cats.data._
  import cats.implicits._
  import cats.instances.future._
  
  for {
    data &lt;- OptionT(repo.getData(id))
    ... 
  }
  </code></pre>
  
  <p>The compiler error I am getting: </p>
  
  <pre><code>could not find implicit value for parameter F cats.Functor[scala.concurrent.Future]
  </code></pre>
  
  <p>This <a href="http://enear.github.io/2016/04/11/monad-transformers/" rel="noreferrer">recent example</a> suggests that this is (was?) possible.</p>
  
  <p>so do the docs in the <a href="https://github.com/typelevel/cats/pull/304/files#diff-90633fbe11dc38a796351af250106b42" rel="noreferrer">pull request for adding <code>OptionT</code></a></p>
  
  <p>and the <a href="http://typelevel.org/cats/typeclasses/functor.html" rel="noreferrer">cats <code>Functor</code> docs</a></p>
  
  <p>What am I missing here?</p>
  
  <p>Thank you</p>