CommonMark Tests (markedjs#1160)

Add tests from CommonMark spec

Author: joshbruce

package-lock.json

@@ -31,7 +31,9 @@
     "eslint-plugin-standard": "^3.0.1",
     "front-matter": "^2.3.0",
     "glob-to-regexp": "0.3.0",
+    "html-differ": "^1.3.4",
     "jasmine": "^3.1.0",
+    "jasmine2-custom-message": "^0.9.0",
     "markdown": "*",
     "markdown-it": "*",
     "showdown": "*",