Support combining files across multiple roots

[ericf]

It would be useful to support combining files which reside in different root paths in one HTTP request.

app.get('/js', combo.combine({rootPath: [
    __dirname + '/public/js',
    __dirname + '/shared/js'
]}), function (req, res) {
    res.send(res.body, 200);
});

http://example.com/js?models/photo.js&widget/lightbox.js

One tricky aspect of this is having ambiguous file paths when a file of the same path resides in both roots.