Support `.YAML` preamble.

[RokeJulianLockhart]

As Cimbali/markdown-viewer#97 (comment) explains. An example is:

---
Key_0: "Value" # Comment
Key_1: "Value" # Comment
- Key_2_0_0: "Value"
  Key_2_0_1: "Value"
- Key_3_1_0: "Value"
  Key_3_1_1: "Value"
---

Content of the document.