Consolidate GeoJSON feature stream I/O

[geowurster]

In progress. Feedback welcome.

@sgillies This is a rough draft (without unittests) of #251 that implements a new fiona.collections.FeatureStream() file-like object for working with GeoJSON feature streams. If it is decided that this object doesn't really belong in Fiona, this about how I planned on implementing Toblerity/fio-buffer#4, so feel free to shoot this down.

Generate some sample data

$ fio cat tests/data/coutwildrnp.shp > features.geojson

Inspect it with Python

import fiona
with fiona.open('features.geojson', sequence=True) as src:
    print(next(src))

RS prefixes are supported as well as reading from stdin and writing to stdout:

import fiona
with fiona.open('-', sequence=True) as src, fiona.open('-', 'w', sequence=True, use_rs=True) as dst:
    for feat in src:
        dst.write(feat)

[sgillies]

@geowurster Let's develop an IO-like class for GeoJSON streams in a different project. It won't require GDAL/OGR, and might get wider use if it didn't entail GDAL. Additionally, there are a few things to figure out yet about feature sequences (geojson/draft-geojson#64 for reference), and we can go faster and more recklessly outside of Fiona :)

[geowurster]

@sgillies Sounds good. I'll drop this in fio-buffer for now but feel free to find me when you get a repo set up for general purpose GeoJSON IO.

[geowurster]

@sgillies Would it be helpful if I worked this into a PR for py-feature-sequences to start a discussion?

[sgillies]

@geowurster 👍

https://github.com/geojson/py-feature-sequences is the new repo. I'll make sure you've got write perms.

[geowurster]

@sgillies I'm in. Thanks!