Allow use of sourceRoot option for source map generation

Hi,

I'm using grunt-contrib-requirejs to build my app. The issue is that the build directory sits next to the app directory. This generates sources that are relative to the `baseUrl`, which causes issues when viewing the folder structure in Chrome.

I don't know if this pull request covers all the bases, but it works for now and I'd be happy to fill in the gaps where needed to get the `sourceRoot` option to work.