Add support for TextSpans

[fibbers]

Fixes issue #12.

Experimented locally with this code which rendered the expected result:

Text.rich(TextSpan(
  text: 'Some text',
  children: [
    TextSpan(text: ' and some more text')
        .bold()
        .textColor(Colors.yellow),
  ],
)),

Also, much is copied from the Text extension, since they both use TextStyle.
However, Text has direction/alignment which TextSpan does not.

[ReinBentdal]

This looks great. Since there is no reason to have the extension named, the extension name can be omitted. I just did this for Text and Icon.
What i mean is extension StyledTextSpan on TextSpan => extension on TextSpan. Also the file has to be exported in styled_widget.dart however this is something i can do aswell.

[fibbers]

Oh, I'm sorry, I think you already made the changes (not that familiar with suggested changes on github).
However, just pushed the requested changes and I added an example on the README, just in case... not sure if that's ok :)

[ReinBentdal]

Nice! Its good you added the example. The documentation is quite lacking at this moment