Gulp data compatibility

Adds gulp-data compatibility.

[gulp-data](https://www.npmjs.org/package/gulp-data) adds a `data` attribute to the file object.

This patch checks for the attribute and merges the passed in `data` and `file.data` objects before passing the template to handlebars.

Please check line 5 of index.js. I've used `_extend` from node's `util`module. Could just as easily use underscore but I didn't want to add another dependency for just one method.