Add default export

[pedroapfilho]

Right now, the only way to add use the the topojson-client is to import it like this:

import * as topojson from "topojson-client";

What if we export it in a way that we can use it like:

import topojson from "topojson-client";

[mbostock]

No. If you want to import everything, use * to import the namespace. Otherwise, import specific symbols, e.g., import {feature} from "topojson-client";. I don’t think it’s appropriate to make the namespace itself a symbol.