Expose style for links

[nikodembernat]

Currently all colors by default are blue without the option to override that:

flutter_text_styled/lib/flutter_text_styled.dart

Lines 252 to 256 in 285f25e

	case TAGS.link:
	style = style.copyWith(
	decoration: TextDecoration.underline, color: Colors.blue);
	link = value;
	break;

It would be great if we could override this value. If you don't have time I can make a PR for that

[nikodembernat]

@dudeck Could you take a look at #6?