When target file is a JSON file, give the option to "merge" all found targets

[lillem4n]

For example if bar.json is requested and a file is found in application/node_modules/foo/bar.json:

{
  "a": 100,
  "b": "maybe"
}

And another, higher priority, file is found in application/bar.json:

{
  "a": 250
}

The result from fs.getMergedPathSync('bar.json') will be:

{
  "a": 250,
  "b": "maybe"
}