Do not fail when the destination file doesn't exist yet / Output success message

* Curretly this plugin fails when the destination JSON file doesn't exist yet. so I fixed this problem by removing [`grunt.file.isFile`](http://gruntjs.com/api/grunt.file#grunt.file.isfile).
* I added the success message of file output. All grunt-contrib plugins and many third-party plugins output the message when they create files to the destination.