Jump to root require if module is not found from the local map

Ok, another take on https://github.com/substack/node-browserify/issues/341

With this it shouldn't matter in which order multiple bundles are added
to the dom. If local require fails to find the module from its map it
jumps the to current root require in the global scope and works its way
back down from there.

Example usage https://github.com/epeli/lazy-browserify/blob/master/both.js