Add a fluent api to work with fg/bg colors and modifiers

[KennedyTedesco]

Partially closes: #109

With this feature, instead of:

Span::styled('[q]', Style::default()->fg(AnsiColor::Red))

We just:

Span::fromString('[q]')->red()

Next step (a new PR): I'll implement a parser, something like:

Line::parse('<fg=green options=bold,italic>Hello</> World')

[dantleech]

🎉 nice!