Generate sourcemaps

What would it take to build support for inline sourcemaps?  I imagine being able to do something like this:

``` js
convert = require("css-whitespace")
cssCode = convert(whitespacedCode, {sourcemap: true})
```

Unless I'm missing something, having sourcemaps in `css-whitespace` would be the only way for rework to support sourcemaps end-to-end.
