Add more html color support

[jdbodyfelt]

This is mainly in reference to the color selection, whose options seem set at:

pandas-erd/pandaserd/module.py

Line 30 in e13d3b0

bg_colors = ['lightblue', 'skyblue', 'pink', 'lightyellow', 'grey', 'gold']

Looking further, these are used in an html tag @ L37. It seems that a modern CSS color list could be used here, no? Or hexadecimal color encoding?

The only caveat I see is if that limit is from the digraph rendering or GraphViz, but those typically have RGB settings as well.
Anyhow, happy to make a PR and add that list.