title: Getting model attributes from pipeline
body: <p>I typically get <code>PCA</code> loadings like this:</p>
  <pre><code>pca = PCA(n_components=2)
  X_t = pca.fit(X).transform(X)
  loadings = pca.components_
  </code></pre>
  <p>If I run <code>PCA</code> using a scikit-learn pipeline:</p>
  <pre><code>from sklearn.pipeline import Pipeline
  pipeline = Pipeline(steps=[    
  ('scaling',StandardScaler()),
  ('pca',PCA(n_components=2))
  ])
  X_t=pipeline.fit_transform(X)
  </code></pre>
  <p>is it possible to get the loadings?</p>
  <p>Simply trying <code>loadings = pipeline.components_</code> fails:</p>
  <pre><code>AttributeError: 'Pipeline' object has no attribute 'components_'
  </code></pre>
  <p>(Also interested in extracting attributes like <code>coef_</code> from pipelines.)</p>