Changed name of set_text_color() to choose_text_color() to avoid confusion with the global option setter functions, e.g. set_text_size(). Updated tests to reflect modified threshold.

Uses the algorithm mentioned at https://stackoverflow.com/a/3943023/2022615 based on the luminance of the background color to determine black or white text. The threshold favored black text in a way that didn't look good with our plots, so I increased the threshold from 0.179 to 0.333.