Added optional coordinateConversion method

Example usage (for GlobalMercator, see https://gist.github.com/tadiraman/12f850ad5a6bdcdc59fb):
```
var mercator = new GlobalMercator();
var converter = geojson2svg(viewportSize,
   {coordinateConversion: mercator.LatLonToMeters});
```

This can be used to generate SVG with correct projection to match web maps.