Add plot(FFTrees, grayscale = TRUE) argument to allow printing to grayscale

[ndphillips]

Suggestion by Guido Williams:

Dear Nathaniel,

I have a question regarding the plot functions in the FFT package. Is there a simple way to print all plots/output in black and white/greyscale? For instance: plot(heart.fft, color = FALSE) or is there another way? This would be very convenient for publication purposes.

[ndphillips]

My reaction: There is no out of the box way to do this. It should be an easy addition though. Would just need to override the icon color arguments.

[ndphillips]

Addressing in https://github.com/ndphillips/FFTrees/pull/210