[hierarchy] add render prop to <Tree /> + <Cluster />

[hshoff]

More control over rendering. Fix for #162

<Tree root={someHierarchy}>
  {({ links, descendants, data, root }) => {
    // up to you
  }}
</Tree>

cc @techniq

[techniq]

👍 thanks. I'll try to take a look at this with react-move in the next few days and let you know how it goes. I'm going out of town this weekend so it might be early next week before I get a chance.