Support styling options / colors for borders and separators

[sergiimk]

Currently looks like it's impossible to stylize borders and separators. Since those are also stored as char it's also impossible to to directly inline ANSI colors in those when creating a custom TableFormat.

Would be great if FormatBuilder supported had API like .border_style(Attr::Dim).

[divi255]

+1. very interested in this feature as well

[pinkforest]

Feel free to send a PR :)

Also keen to get the term switched with termcolor or yansi or owo-colors or crossterm if you're up to it.

Thinking that the whole color thing should be feature-gated but as default-on.