Added support for `QuantitiveScale.nice` argument: `count`

See [the documentation](https://github.com/mbostock/d3/wiki/Quantitative-Scales#wiki-linear_nice) for details.

Tested in my codebase and works well.
Merge pull request #1292 from drewnoakes/patch-1

Added support for d3 QuantitiveScale.nice argument: count