Support for source maps

This stuff is great, we are using it for our [Sky.com](//www.sky.com) homepage and services sites.  There is just one feature we wanted to add which was support for [source maps](http://www.html5rocks.com/en/tutorials/developertools/sourcemaps/) for ease of debugging.  I have explained about this in the `README.md` which I have copied and paste below.  I have updated the version to `2.1.0` however you are welcome to change this. :) BF

#### Source Maps

The task will ensure that any source map for `.css` or `.js` file is revisioned with the same revision as the source file.

For example, `js/main.js` revisioned to `js/main.9d713a59.js` will also have `js/main.js.map` revisioned to the same hash `js/main.9d713a59.js.map`.