map_coords for tuples

[JanLikar]

I think it would be very useful to have a function that would work similar to the geojson.utils.map_coords, but would call the mapping function with tuples of coordinates.

This would, for instance, enable writing simple converters between projections.

[dharasty]

I came to this same conclusion less than 10 minutes after using geojson.

I'd be happy to write this.

[frewsxcv]

There's a PR open for this, but the tests are currently failing, in case anyone is interested in picking it up

[JanLikar]

See #112.