New `data_area_fill_color` parameter for `nanoplot_options`

[JoeSydlowski]

Prework

• Read and abide by gt's code of conduct and contributing guidelines.
• Search for duplicates among the existing issues (both open and closed).

Proposal

A parameter for data_area_fill_color along the lines of reference_area_fill_color under nanoplot_options would be nice to have! The data area here doesn't look quite right, but I don't want to hide it with show_data_area if I could change the color instead.

[rich-iannone]

Hey @JoeSydlowski , I followed your suggestion and added the option. Try it out! And let me know if there's anything else that could be done.