ENH: labelFormat to support numeraljs format

[tejesh0]

Numeraljs library has to be installed by the developer just like d3.

[susielu]

This is already available if you pass a custom function into labels, you would just do something like this:

var legend = d3.legendColor()
    .labels(({i, generatedLabels}) => numeral(d).format("0a"))
    .useClass(true)
    .scale(thresholdScale)