title: How to get sample indices from RandomUnderSampler in imblearn
body: <p>Does anyone know if/how one can get the indices of the selected samples after undersampling with imblearn's RandomUnderSampler?
  There used to be the argument "return_indices=True" which was now removed for the new version and supposingly was replaced with an attribute "sample_indices_". However, if I try to use that attribute, it doesn't work (see code below). I'm using imblearn version 0.6.2.</p>
  
  <pre><code>russs = RandomUnderSampler(random_state=0,sampling_strategy={6: 600}).fit(X_train_point,y_train_point)
  russs.sample_indices_
  
  AttributeError                            Traceback (most recent call last)
  &lt;ipython-input-78-8397ba40f19b&gt; in &lt;module&gt;
        1 russs = RandomUnderSampler(random_state=0,sampling_strategy={6: 600}).fit(X_train_point,y_train_point)
  ----&gt; 2 russs.sample_indices
  
  AttributeError: 'RandomUnderSampler' object has no attribute 'sample_indices'
  </code></pre>