Change relSourceMap and absSourceMap into a single mapSourceURI option

Also create proper source maps for overridden sources (fixes #758)
This deserves a comment saying that this one takes care of overridden files. At first sight it seems redundant with the one that is commented just above. Similarly, it took me a while to realize that this line actually took care of overridden files: I thought you forgot to handle them, despite introducing the feature that allows the fix ^^.