Curves should use named parameters.

[mbostock]

For consistency with d3/d3-ease@f0ea8b5 (though the ease.ease part was later reverted) and d3/d3-interpolate@70ebb63, curves should also use named parameters rather than binding optional parameters in line.curve and area.curve. Specifically, code like this:

var line = d3.line().curve(d3.curveCardinal, 0.5);

Should be replaced with:

var line = d3.line().curve(d3.curveCardinal.tension(0.5));