Add option to skip category labels

[tvolkert]

While this change to add category labels (#1610) makes sense on api.dartlang.org, it doesn't make sense on docs.flutter.io. e.g. see the presentation therein:

To the Flutter developer, those categories don't add any value and actually add confusion.

It'd be nice to have a flag to pass to pub global run dartdoc that specified to not use the categories for Dart.

/cc @kevmoo @jcollins-g