Combining features from GFM and Extra?

Is there an obvious way to combine features from `GithubMarkdown` and `MarkdownExtra` that I'm missing?

Specifically, I'd like to have GitHub-flavored markdown plus one or two features from Markdown Extra, mainly special attributes for headers, but this appears to be hard coded into MarkdownExtra rather than available as a trait?

Is there any practical reason why the implementations don't follow the same pattern of applying features as traits through and through? The documentation seems to suggest the idea that you could take features from different implementations and combine then in your own implementation, but maybe the traits were simply a means of sharing certain features between implementations internally?