Add inline sourcemaps support

Sourcemaps as a separate file (e.g., `main.css.map`) are a yet-to-be-solved architectural shortcoming of Broccoli. However, inline sourcemaps—base64 encoded at the end of the file—are simple to support, as is, with Autoprefixer and PostCSS.

The change to `README` should explain the usage pattern. Strangely, PostCSS requires a `from` and `to` in order to make this work, even though it doesn't require a name for the file it is processing. Thankfully, `broccoli-filter` passes a name to us (`relativePath`).

This work should supersede #1, so I would appreciate the review of @SlexAxton and @myfreeweb.
Just use [object-assign](https://github.com/sindresorhus/object-assign) at the top instead.