add remark lint to ensure fenced codeblocks are formatted properly.

.remarkrc

@@ -0,0 +1,6 @@
+{
+  "plugins": [
+    ["remark-lint-code-block-style", "fenced"],
+    ["remark-lint-fenced-code-flag"]
+  ]
+}

package.json

@@ -17,6 +17,8 @@
     "husky": "^0.14.3",
     "minimist": "^1.2.0",
     "nugget": "^2.0.1",
+    "remark-cli": "^4.0.0",
+    "remark-preset-lint-markdown-style-guide": "^2.1.1",
     "request": "^2.68.0",
     "standard": "^8.4.0",
     "standard-markdown": "^4.0.0",
@@ -48,7 +50,7 @@
     "lint-js": "standard && cd spec && standard",
     "lint-cpp": "python ./script/cpplint.py",
     "lint-py": "python ./script/pylint.py",
-    "lint-docs": "npm run lint-js-in-markdown && npm run create-typescript-definitions",
+    "lint-docs": "remark docs -qf && npm run lint-js-in-markdown && npm run create-typescript-definitions",
     "lint-js-in-markdown": "standard-markdown docs",
     "create-api-json": "electron-docs-linter docs --outfile=out/electron-api.json --version=$npm_package_version",
     "create-typescript-definitions": "npm run create-api-json && electron-typescript-definitions --in=out/electron-api.json --out=out/electron.d.ts",