Add json-seq option to rio shapes.

See draft-ietf-json-text-sequence-04

  http://tools.ietf.org/html/draft-ietf-json-text-sequence-04

for a description of JSON text sequences. In a nutshell, this
means a feature collection as a LF-seperated sequence of feature
objects that can be parsed and processed incrementally. Is this
the first implementation in a geographic application? I haven't
seen others yet.