title: F1-score per class for multi-class classification
body: <p>I'm working on a multiclass classification problem using python and scikit-learn. Currently, I'm using the <code>classification_report</code> function to evaluate the performance of my classifier, obtaining reports like the following:</p>
  
  <pre><code>&gt;&gt;&gt; print(classification_report(y_true, y_pred, target_names=target_names))
               precision    recall  f1-score   support
  
      class 0       0.50      1.00      0.67         1
      class 1       0.00      0.00      0.00         1
      class 2       1.00      0.67      0.80         3
  
  avg / total       0.70      0.60      0.61         5
  </code></pre>
  
  <p>To do further analysis, I'm interesting in obtaining the per-class f1 score of each of the classes available. Maybe something like this:</p>
  
  <pre><code>&gt;&gt;&gt; print(calculate_f1_score(y_true, y_pred, target_class='class 0'))
  0.67
  </code></pre>
  
  <p>Is there something like that available on scikit-learn?</p>