added ability to accept its data from file.data

I've created a de-coupled (generic) data source plugin that sets a data attribute to the file object. Through the new ```gulp-data``` plugin the data attribute can be set from a variety of sources such a JSON file, front-matter, a database, anything really. See https://www.npmjs.org/package/gulp-data for more detail and the rationale behind it. For this to be effective, I'm asking plugin devs that receive data through the options parameter to also accept this data via the file.data property. I've made the change and have added a test. Thanks!