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. The jade plugin remains agnostic to how that data object gets set, but it's a good proof of concept for how this can be standardized. 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
at this point, you might want to change this to be something like ```opts.data = _.extend(file.data, opts.data)``` in case you want to merge data rather than completely overwrite it. Up to you.