add nest.apply function

I sometimes need a way to modify {key: id , values : [] } nested objects. 

usage : 
yields = [{yield: 27.00, variety: "Manchuria", year: 1931, site: "University Farm"},
              {yield: 48.87, variety: "Manchuria", year: 1931, site: "Waseca"},
              {yield: 27.43, variety: "Manchuria", year: 1931, site: "Morris"}]

nest = d3.nest()
    .key(function(d) { return d.year; })
    .key(function(d) { return d.variety; })
    .apply(function(o,depth) {o.text = 'title : ' + o.key ; })
    .entries(yields);

Cheers
C.