Allow for manual CSS style rules in style= arg of tab_style()

[rich-iannone]

While we can add quite a few cell styles using style.text(...) and style.fill(...) there may be CSS styles that would be useful to have in a table but don't have a dedicated argument. For this reason, it would be good to be able to add CSS rules as strings (e.g., "font-variant: small-caps;").