Allow easier customizing of the y-axis formats

This is especially useful if you have a line
and bar chart with two data series with extreme differences.
Such as:

    data_y1 = [10, 13, 14, 20]
    data_y2 = [0.0003, 0.0032, 0.0057, 0.0081]

In the above case one may one would like to format the first
y axis with:

        cart.y1Axis
                    .tickFormat(d3.format(',f'));

and the second y axis with:

        chart.y2Axis
                    .tickFormat(function(d) {
                     return d3.format(',0.4f')(d) });

This is now possible with passing a keyword ```yaxis2_format```
when creating a new ```linePlusBarChart`:

    chart = linePlusBarChart(name="linePlusBarChart",
                             width=500, height=400, x_axis_format=None,
                             yaxis2_format="function(d) { return d3.format(',0.4f')(d) }")
Merge pull request #65 from oz123/develop

Allow easier customizing of the y-axis formats