Emit Shapefile header

This adds an additional  `header` event to shapefile streaming.  Sometimes it's useful to know the bounding box (`bbox`) of the shapefile before you start streaming features.   In my case, I wanted to create images from shapefiles (pretty much [what you're doing in the us-atlas project](https://github.com/mbostock/us-atlas/blob/master/bin/rasterize#L22)) but I needed to [project to a bounding box](http://bl.ocks.org/mbostock/4707858) before the features started streaming in.