title: sklearn agglomerative clustering with distance linkage criterion
body: <p>I usually use scipy.cluster.hierarchical linkage and fcluster functions to get cluster labels.
  However, the sklearn.cluster.AgglomerativeClustering has the ability to also consider structural information <em>using a connectivity matrix</em>, for example using a knn_graph input, which makes it interesting for my current application.</p>
  
  <p>However, I usually assign labels in fcluster by either a 'distance' or 'inconsistent' criterion, and AFAIK the AgglomerativeClustering function in sklearn only has the option to define the number of desired clusters (so criterion='maxclust' in the scipy library).</p>
  
  <p>I am wondering whether it is possible to simply return the linkage matrix from the AgglomerativeClustering in this case to take advantage of both libraries functionalities?</p>
  
  <p>Thank you</p>