Callback to limit parts of the file to preview

[madduck]

trafficstars

I inject Markdown into mails I write. As a consequence, when I call markdown-preview, I get a preview that includes RFC5322 headers, and a mail signature, both of which are not Markdown.

Would it be possibel to give me a config option such that I can define a callback function that takes the entire file and only returns the parts that I want to be previewed?