allow dashes within yaml front matter

This module doesn't work with the following front matter. The first 3 dashes in the markdown table header get used as the end of the front matter. This pull request fixes that by requiring a preceding newline character prior to the dashes that represent the end of the front matter.
```
---
designCommentary: |
    | Viewport Width | 0px | 321px | 600px | 800px | 1000px |
    | :------------: | :-: | :---: | :---: | :---: | :----: |
    | .col-2 | 1 column | → | 2 columns | → | → |
    | .col-3 | 1 column | → | → | 3 columns | → |
    | .col-4 | 1 column | 2 columns | → | 4 columns | → |
    | .col-6 | 1 column | 2 column | 3 columns | → | 6 columns |

    Gutters/Margins on the outer edges of the site container are subject to
    change, but the site container currently occupies:

    - 90% of the viewport width at 799px and below
    - 95% of the viewport width at 800px and above
---
```