Add feature for coodinate reprojection

[jdavidls]

I think coordinate reprojection can be usefull if we want to use the resulted geojson data with other services like google places/routes, or other openstreetmap open services.

I think the most simple implementation for that can be throught a callback, like this:

var result = osmtogeojson(osmdata, {
  reprojection: (coords) => {
    // reprojection here
    return otherCoords;
  }
});

Would a pull request be accepted?