Adds get_input_text() that can be used to get the literal variable name and make_named_data() that creates a list with the input data frames. This second one is necessary because the inputs may be quosures and we need to get the underlying data frames. Default names are $x and $y.

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.

Currently only used for setting global text and title font sizes, but can be extended to other options that we may wish to set for more than one plot. For fonts, we now provide `set_font_size()` that can be used to set text and title sizes across all subsequent animations. Includes internal getter functions get_{text|title}_size().

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.