fixes-336: Extend the Ticker interface to allow passing a format function

[DavidGamba]

Fixes #336 This allows for easy wrappers around the plot.DefaultTicks struct with custom format functions:

    type myCustomFormatTicks struct{}

    var _ plot.Ticker = myCustomFormatTicks{}

    func (myCustomFormatTicks) Ticks(min, max float64, format func(v float64, prec int) string) (ticks []plot.Tick) {
      return plot.DefaultTicks{}.Ticks(min, max, myFormatFloatTick)
    }

    func myFormatFloatTick(v float64, prec int) string {
           return strconv.FormatFloat(floats.Round(v, prec), 'g', 8, 64)
    }

    p.Y.Tick.Marker = myCustomFormatTicks{}

[DavidGamba]

@sbinet please take another look.

[sbinet]

@kortschak @eaburns any feedback about this change?

[DavidGamba]

@sbinet please take another look. I apologize for the delay... With this, to provide a format to the default ticks, all I have to do is:

...
	p, err := plot.New()
	if err != nil {
		return err
	}
	p.X.Tick.Format = myAxisTickFormat
	p.Y.Tick.Format = myAxisTickFormat
...

func myAxisTickFormat(v float64, prec int) string {
	return strconv.FormatFloat(floats.Round(v, prec), 'f', 0, 64)
}

[DavidGamba]

@sbinet PTAL