Accept a "data" property as additional metadata

I'm developing a standard methodology for passing meta data to plugins using a separate plugin called ```gulp-data``` (see https://www.npmjs.org/package/gulp-data). It all works fine without this change, but when writing gulp plugin unit tests by instantiating a File object with the data property attached, the tests fail. It looks like setting a default value here fixes that. Now I can write tests on adhoc files:

```
var srcFile = new gutil.File({
  path: "test/fixtures/filepath.txt",
  cwd: "test/",
  base: "test/fixtures",
  contents: new Buffer("Fake data"),
  data: { message: "Some data" }
});
```