title: treemap aggregate values which are too small to be visualized correctly
body: <p>so I've started using d3 treemap, and of course it's great.<br>
  problem is- some of my data has very large size, and some very small size (i.e <code>{ name: 'j', size: 5000} , { name: 'x', size: 15}</code> etc...).<br>
  So the result visualization of course shows several very large cells and some very small ones.<br>
  on the small cells, the text is not visible at all, so they're not very usable.  </p>
  
  <p>I was wondering if there's an option to aggregate all small value cells to one 'others' cell?  </p>