Fix #37; add topojson.feature.

This deprecates topojson.object, replacing it with a method that returns proper
GeoJSON rather than a hybrid between TopoJSON and GeoJSON.
Rewrite object tests for feature.
Rewrite bin/geojson to use topojson.feature.
Remove straggling usage of topojson.object.
Restore support for --id-property.
Remove topojson.object; bump to 1.0.0.

This wouldn’t have been backwards-compatible anyway because it changed the
behavior of topojson.object, removing propagation of id and properties. Rather
than drag the old method along, now seems as good a time as any to call this
version 1.0.0 of the client API reference—there are quite a few users already!
Merge branch 'feature'