Allows a user to specify an element to bind to

Allows a user to specify an element to bind the chart too. This allows multiple charts on a single page

Use case will be:

    var lineChart = c3.generate({
        data: {
            columns: [
                ['data1', 30, 200, 100, 400, 150, 250],
                ['data2', 50, 20, 10, 40, 15, 25]
            ]
        },
        target: "#lineChart"
    });

    var pieChart = c3.generate({
        data: {
            // iris data from R
            columns: [
                ['data1', 30],
                ['data2', 120],
            ],
            type : 'pie',
        },
        target: "#pieChart"
    });