add color hex code labels to bi_pal()

[torreyma]

When running bi_pal() to check my palette, it would be helpful to have an option to see the color hex codes printed on top of the palette tiles, like this:
custom_pal3.plot_biscale_palette.pdf

Describe the solution you'd like
I think this is pretty easy to implement. Just add a boolean on/off switch to bi_pal() and then a line like:
ggplot2::geom_text(data = leg, ggplot2::aes(x = x, y = y, label = bi_fill)) +
to the ggplot call in bi_legend_build().

Describe alternatives you've considered
I implemented this as standalone code. I can't think of a reason it shouldn't be part of the biscale package itself. But maybe I'm missing something?