Add IntTicks

[isa9660]

Nice to meet you. Thank you for a wonderful library.
In order to easily represent integers, I created 'IntTicks'.
By the way, if you change 'DefaultTicks' in 'Custom tick marks' of example to 'IntTicks', it will have the same result as the example.
('1,.5e,+04'->'15,000','1e,+04'->'10,000)

[DavidGamba]

Take a look at #362.
With it, you should be able to do:

    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), 'f', 0, 64)
    }

    p.Y.Tick.Marker = myCustomFormatTicks{}

And get the same results.

[isa9660]

I had planned from before.
Even now I can do it custom, so I am not in trouble,
I will close it. Thank you very much!