title: How to add multiple joinCondition in vis.js while clustering
body: <p>I am using vis.js for network diagramming, I need to cluster some nodes. 
  For Example: create new cluster when a node property is 'aaa' (Suppose two nodes has 'aaa' they will create cluster). 
  Again create new cluster when a node property is 'bbb' (Suppose two nodes has 'bbb' they will create another cluster).</p>
  
  <p>I have two problems:
  1) I don't want to hard-code 'aaa' or 'bbb'. 
  2) I don't want to create multiple clusterOptionsByData object and invoke network.cluster(clusterOptionsByData) multiple times.
  Is there any way to pass multiple joinconditions while clustering in  vis.js?</p>