Expose namespaces as d3.namespace in standalone.
Expose namespaces as d3.namespace in standalone.

Since I’m not planning on exporting any methods related to namespaces in the
standalone API, it seems simpler to just export the map of prefixes.

d3/d3-selection,GerHobbelt/d3-selection,jimkang/d3-selection,chrisprice/d3-selection