Fix possible file limit errors

As we currently may need higher nofile limits than usual/default on
various systems this commit should probide a fix for that an allow to
build HackMD without highering these limits and increase security.

Inspiration was found in a copy-webpack-plugin-issue[1] and found by
@thegcat[2]. Thanks for that!

Signed-off-by: Sheogorath <sheogorath@shivering-isles.com>

[1]:
https://github.com/webpack-contrib/copy-webpack-plugin/issues/59#issuecomment-228563990
[2]: https://github.com/thegcat

package.json

@@ -42,6 +42,7 @@
     "font-awesome": "^4.7.0",
     "formidable": "^1.0.17",
     "gist-embed": "~2.6.0",
+    "graceful-fs": "^4.1.11",
     "handlebars": "^4.0.6",
     "helmet": "^3.3.0",
     "highlight.js": "~9.9.0",