Supporting a new gulp-useref workflow.

I have a [proposed change](https://github.com/jonkemp/gulp-useref/pull/3) over on [gulp-useref](https://github.com/jonkemp/gulp-useref) that makes the workflow a bit more flexible and more gulp-like. For reference, the discussion actually [started](https://github.com/jonkemp/gulp-bundle/pull/5) over on [gulp-bundle](https://github.com/jonkemp/gulp-bundle).

The change here make the Yeoman gulp workflow a little easier to understand (in my opinion) and a bit more gulp-like.

This will have to wait for the pull request to go through on gulp-useref and for a new version of that plugin to be released, but I wanted to at least allow opportunity for discussion both here and on the useref plugin in case anyone sees opportunities for improvement.