First stab at d3.geo.clipView.

This is a simple stream transformation for the moment.  Note that the
clip region points should be specified in counterclockwise order as
required by d3.geom.polygon, and unlike GeoJSON, the first and last
points should not be coincident.

This is a work-in-progress; lines are not intersected at the moment!

[Example](http://bl.ocks.org/jasondavies/5082889).