Document with literal `---` or single horizontal rule at top is mangled

## Bug

If a document with `---` at the top is provided, an extra `---` will be written to the document because the leading `---` is detected as a frontmatter.

## Reproducer

```md
---

Example!
```

## Expected Result 
```md
---

Example!
```

## Actual Result

```md
---
Example!
---


```
Trailing newlines intended; see #180