sort json file by keys

[tlightsky]

#390

[gabrielrobert]

@tlightsky @gilmarsquinelato It looks like this PR is causing problems for nested content. You can reproduce the problem easily:

With this PR:

var data = { nested: { content: 'will be removed' } };
JSON.stringify(data, Object.keys(data).sort(), 2);

Result:

"{
  "nested": {}
}"

Before this PR:

var data = { nested: { content: 'will be removed' } };
JSON.stringify(data, null, 2);

Result:

"{
  "nested": {
    "content": "will be removed"
  }
}"

[tlightsky]

true,
maybe we should switch to this:
https://www.npmjs.com/package/json-stable-stringify
from:
https://stackoverflow.com/a/16168003