Should handle strong emphasis correctly

[kellyjosephprice]

https://github.github.com/gfm/#emphasis-and-strong-emphasis

> JSON.stringify(window.RDMD.mdast("***strong** emphasis*"), null, 2)
"{
  "type": "root",
  "children": [
    {
      "type": "paragraph",
      "children": [
        {
          "type": "strong",
          "children": [
            {
              "type": "text",
              "value": "*strong"
            }
          ]
        },
        {
          "type": "text",
          "value": " emphasis*"
        }
      ]
    }
  ]
}"