support i/o to topojson format

Topojson is pretty handy for data visualisation when working with adjacent polygons (think- counties in the state/country). While there is a separate topojson package (https://github.com/calvinmetcalf/topojson.py), I am pretty sure geopandas will benefit from direct I/o functionality. I will be happy to PR, but will be thankful for the suggestions first.

1. do you feel fine with adding topojson module as an optional dependency??
2. what will be the best way to use topojson?

2.1

with open('file.json', 'w') as f:
    df.to_json(f, topo=True)

2.2

with open('file.json', 'w') as f:
    df.to_topojson(f)

as for reading, it seems reasonable to try parse topojson if geojson parsing fails?

[jorisvandenbossche]

Documentation about supported formats is indeed sparse. PR to improve that certainly welcome!

Normally I would say: the support depends on fiona, but even in the fiona.supported_drivers I don’t see topojson being mentioned (although that might be because until recently gdal support was under the geojson driver:http://www.gdal.org/drv_topojson.html).

If fiona supports reading, it might also support it for writing.

[Casyfill]

just realized topojson is supported for reading (read_file), while I found no documentation on that.