Add getFromThemeData method to Style

[tneotia]

Fixes #303

Users can do

Html(
   style: Style.getFromThemeData(ThemeData.of(context))..addAll(/*other styles*/),
),

To make h1-h6 and other text elements behave more in line with Flutter styles rather than CSS styles.

[erickok]

I like this approach. Would just rename the method to Style.fromThemeData(...).

And similarily we can maybe do a Style.fromCss(...) which would allow you to pass in a css string?

[tneotia]

Would just rename the method to Style.fromThemeData(...).

Done.

And similarily we can maybe do a Style.fromCss(...) which would allow you to pass in a css string?

Nice idea, I added this in #655 since it has the upgraded CSS parsing.