Improve docs

[mattijn]

This is a big documentation PR.

It is focused on

• https://mattijn.github.io/topojson/example/input-types.html
• https://mattijn.github.io/topojson/example/settings-tuning.html

Once this PR is merged, the pages of these urls will be updated.

While making this documentation I fixed a bug in the prequantize function. This is included in this PR.
Also improved some errors during simplifying.

The documentation now reflects the code in the master. I did not check if there is a difference compared to the latest released version, but the difference should be minimal, if any.

[mattijn]

After holiday will continue with

• https://mattijn.github.io/topojson/example/output-types.html
• https://mattijn.github.io/topojson/how-it-works.html