plot_indicator() to allow a second line

[drnickisaac]

Plotting two lines together might be a desirable and even common practice, e.g. the MSI typically plots the smoothed and unsmoothed versions of the same indicator. We might also want to plot MSI and BMA on the same axes.

I think this should be straightforward: add arguments "indicator2=NULL, CI2=NULL" into the call for the plot_indicator.

[AugustT]

What if they want to add a third or fourth line?