Support pandoc-compatible front matter

[andrewshadura]

Related to #33, it would be great if ronn could support the front matter pandoc uses:

---
title: COMMAND
section: 1
header: User Commands
footer: command
---

NAME
====

command - short description

SYNOPSIS
========

**command** [*OPTION*]... ...

(for this specific Markdown source it would also need to support this style of headings too)

[apjanke]

This seems like a good idea, but I'm a little leery of adding it without due consideration; ronn is supposed to be a "simple" tool. Let's think about this some. I welcome comments from other users. Leaving it open for thought and discussion; if nobody has a reason why not, I'm inclined to include it.

Marking for potential release at some point in "The Future".

[paravoid]

I'm trying to parse libmaxminddb's manpages; they use pandoc right now. They don't use this preamble format, but do provide their own name section, as well as headers that start from h1, not h2. It'd be neat if this could be supported out of the box, so +1 :)

[apjanke]

I'm hearing user support, so I'll look in to this. Thanks for the feedback!