Improve Defualt Markdown Config and Documentation

[mcmah309]

I tried using this package and found setup difficult with little documentation around how this package works. The https://pub.dev/packages/flutter_markdown just works out of the box and all fields and classes are documented. This packages seems to require some sort of configuration to get styles working correctly. It is not clear how the classes compose to get desired results. It also seems like some features e.g. latex, are in the example rather than the actual package? I spent about 30 minutes trying to figure it out and realized it was not worth the effort. I think the documentation should be improved on how to setup a base looking config and make necessary changes. Also it would be nice if flavors were defaults you could just add in. Like LatexSupport or GithubFlavor.