[TxMt] Supporting YAML front matter in Markdown

[Stefan Daschek]

I’m working on a bundle to support YAML front matter in Markdown (see 
https://github.com/noniq/Markdown-Front-Matter.tmbundle/).

The grammar for this is easy, but I also want to strip front matter 
blocks (if present) from the document before processing the markdown. 
And it would be really cool to have this working with whatever Markdown 
bundle (standard, GitHub Markdown, …) the user is currently using.

So in principle I want to implement a preview command that just strips 
the frontmatter and then delegates to whatever else preview command 
would have been normally used, if this bundle would not be installed 
(like calling `super` in an overridden oop method).

Is this possible?