change default shared_coords from True to False

[mattijn]

This PR changes the default from shared_coords from True to False. The impact is that computed topologies will use by default the strategy of path-connected (shared_coords=False), where before the default was coords-connected (shared_coords=True).
See for more info on the differences in the docs: https://mattijn.github.io/topojson/example/settings-tuning.html#shared_coords.

[mattijn]

The previous default was mainly there because of performance reasons. With #179 by @theroggy the performance differences are nearly identical, giving more preference to a new default.

[theroggy]

fixed #187