Should allow blank headers of depth greater than 1

[kellyjosephprice]

https://github.github.com/gfm/#example-49

> JSON.stringify(window.RDMD.mdast("## "), null, 2)
"{
  "type": "root",
  "children": [
    {
      "type": "heading",
      "depth": 1,
      "children": []
    }
  ]
}"