Fix Issue 202: Second Y-Axis Label

Fix Issue 202: Second Y-Axis Label

Option name is "y2label". This results in a label on the right-hand side
of the chart which is rotated in the opposite way of the label on the
left-hand side. It gets CSS classes "dygraph-label dygraph-y2label" (the
primary y-axis label gets "dygraph-label dygraph-ylabel").