Add fullPaths option.

Completely preserves the original module ids, instead of converting them to numerical indices. Useful for analysing a bundle after it's created, making the browser-unpack module a viable standard approach for picking apart bundle contents.

I suspect there might be some overlap with `opts.commondir` – but I'm unsure if that option still alters the bundle output.

Looking at making [disc](http://github.com/hughsk/disc) a little more future-proof by using browser-unpack, and need this option available to determine the file tree. Thanks! :)