Allow creating `rio.Text` without passing a TextStyle

[mad-moo]

Styling rio.Text always requires instantiating a separate rio.TextStyle. This gets annoying real fast. Add an overload so that one can either pass a TextStyle or pass the styling values directly.

While we're at improving textstyle, consider improving them in general:

• Add shortcuts for bold & italic
• Allow specifying colors as strings, e.g. danger in the TextStyle class