Replace Axis.integersOnly option with maximumFractionDigits

[nickofthyme]

Is your feature request related to a problem? Please describe.
With formatting the Axis tick labels, it is sometimes desirable to alter the values prior to their formatting. This is because the formatting can alter the true numerical value of label to the point where the label is no longer accurate with respect to the axis scale.

Describe the solution you'd like
We added the option to limit axis values to integers in #387, but I think making this more flexible as to define a maximumFractionDigits would provide greater utility to the chart consumer.

This option's naming comes from the Swift documentation for the NumberFormatter, see docs.

Describe alternatives you've considered
There is no true alternate to this other than a way for the chart consumer to manually dictate the ticks to be shown which would be too flexible from a configuration level.

Kibana Cross Issues

• Round axis values with integersOnly when Decimal format is 0 kibana#170151