Support feature count

[stevage]

This works really well, thank you! It would be helpful to have a way of knowing the total feature count, in order to provide percentages to the user when processing files.

I don't know much about how Shapefiles work, but ogrinfo can provide this info instantly, so presumably it's stored in metadata and doesn't require manuallly counting.

[rubillionaire]

Adding this commit https://github.com/mbostock/shapefile/compare/master...rubillionaire:shapefile:master for anyone else looking interested in this. Small change from the above, using getUint32 instead of getUint16 to get the feature count, as referenced in this documentation