Allow for custom xlabel and ylabel in plot_wfdb

[briangow]

It would be nice to allow the user to pass custom labels (xlabel, ylabel) when using the plot_wfdb function. It is possible to customize both x and y labels in the plot_items function already. The plot_wfdb function will automatically populate the x and y labels with the information from the WFDB signal. While this is helpful, users may want to override this default to change formatting (etc) for plots they are making.