title: Find properties of JS object with truthy values using lodash
body: <p>Let's say I have an object like:</p>
  
  <pre><code>var foo = {
      alpha: true,
      beta: false,
      gamma: true
  }
  </code></pre>
  
  <p>I can use _.findKey to get one key with a true value, but I'd really like to get an array containing all keys with a true value. E.g.</p>
  
  <pre><code>_.findAllKeys(foo, function(val) { return val; });
  // yields -&gt; ["alpha", "gamma"]
  </code></pre>
  
  <p>It's simple enough to write a function to do this, but it seems like such an obvious generalization of findKey that I feel I must just be missing it. Does lodash have such a function?</p>