Greetings all,

This patch adds a new option `'mixed' to `org-md-headline-style' that
when enabled allows for exporting the first two headline levels in
Setext-style, and the rest in ATX-style. I like the mixed style for
certain situations, and I couldn't find a way to export in this manner.

I've added what I think is appropriate in the news file and the manual,
but this is my first time doing so, please let me know if this needs any
improvements/tweaks.

Thanks,
Lee