Option to turn of folding for headline tags mark with ---

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Hi Drew I really love your plugin and I'm using it on my daily basis for writing articles in markdown. Since I'm using [Jekyll](http://jekyllrb.com/) for running my blog, I'm also using the YAML settings at the beginning of each markdown file.

```

---
layout: post
title: here goes the article of a new post
meta-description: <---- Here begins the first fold level because of the special headline marks

---

## The kitchen timer <-- here should be the first level of the folding level

...
```

If you have problems with following the example above, please consult the example below:

  1 ---

  2 layout: post

  3 title: On My Way On Becoming a Self-Certified Pomodoro Master

  4 ## meta-description: Motivation for the ongoing series of becoming a Self-Certiefied Pomodoro Master [12 lines]-----------

 17 ## Why? [22 lines]--------------------------------------------------------------------------------------------------------

 40 ## The First Pomodoro [36 lines]------------------------------------------------------------------------------------------

 77 ## Following the six objectives [17 lines]--------------------------------------------------------------------------------

 95 ## Conclusion [9 lines]---------------------------------------------------------------------------------------------------

Cheers

Matthias

Start by inspecting the vim-markdown-folding folding logic and configuration entry points for how YAML front matter and headline tags are recognized. Reproduce the example with Jekyll front matter, then verify that the requested option prevents folding there while preserving folding for normal Markdown headings.

Written by the indexing model from the issue text.