Add basic aabb query to the lib

[Fxlr8]

Hi, It took me some time to find this examlple

// Find the nodes within the specified rectangle.
function search(quadtree, x0, y0, x3, y3) {
  quadtree.visit(function(node, x1, y1, x2, y2) {
    if (!node.length) {
      do {
        var d = node.data;
        d.scanned = true;
        d.selected = (d[0] >= x0) && (d[0] < x3) && (d[1] >= y0) && (d[1] < y3);
      } while (node = node.next);
    }
    return x1 >= x3 || y1 >= y3 || x2 < x0 || y2 < y0;
  });
}

Please add this method to the tree, or at least add it to the README

[mbostock]

Related #18 (comment).

[Fil]

Is the code in https://observablehq.com/d/ff2450667f13132f readable? I tried to comment it in detail.