Add series data as parameter of labelInterpolationFnc

[zedentox]

This PR allows to choose different labels depending on the series.

Chartist.plugins.ctPointLabels({
    labelInterpolationFnc: function(value, serie) {
        if (serie.name === 'series-dollars') {
            return '$ ' + value;
        }
        if (serie.name === 'series-euros') {
            return value + ' €';
        }
        return value;
    }
})

I used grunt default to passe tests and build dist sources.
I tested this feature with a simple Bar chart and a multiline Line chart. I also tested if no errors occurs when no serie names are present. Seems ok to me.

[joelstein]

I have a use case which requires the additional data in the 'data' variable, not just the series. I would recommend passing in data, from which you can easily get the series, index, and other values. For example, I want this so I can show a point label of the percentage increase from the previous item in the series, which would be easy to calculate if I had the whole data variable.