Fix error if a directory exists in the data directory

Error occurs if the directory exists in the data directory.

ex)

Directory structure

````
brackets-snippets
|- config.js
|- data
  |- some.json
  |- someDirectory
    |- some.snippet
````

some.json

````
[
    {
        "name": "somename",
        "trigger": "some",
        "usage": "some",
        "description": "snippet in sub directory",
        "template": "someDirectory/some.snippet"
    }
]
````

Thank you for providing a nice plugin!!
Thanks.