Move path transformation logic into function similiar to gulp.src

[Silic0nS0ldier]

The path transformation logic is great for reducing IO, but comes at the expense of increased RAM usage since it introduces a clog in the stream. We can resolve this by creating our own version of gulp.src that performs this work during the read phase, which will allow us to perform processing of bundles in a more stream like fashion, namely;

• Start processing a bundle the second we have its resources, and emit its result during the same _transform invocation that ultimately triggered it.
• Push all files straight on through with required files cloned (we currently can't due to overwrite logic) so that later processing can run.

Final result should be less used RAM and less read operations.

[Silic0nS0ldier]

Figured out how gulp determines the base path (which directly impacts where files get delivered). Looking much more reasonable getting this implemented however I think it would be better to release it as its own package. A vinyl source with file overriding logic has uses in more than just this library. It may even already exist (which would be awesome if its stable and supported).

[Silic0nS0ldier]

Like so: https://www.npmjs.com/package/vinyl-fs-merge
Will need to be tested, but this would allow substantial performance improvements.

[Silic0nS0ldier]

To simplify testability, the implementation should be split like;

• File resolution (overrides, etc), returns a map of virtual paths to actual paths
• Vinyl source
• Bundle substreams (similar to current system, minus file resolution and waiting for all files to start bundling)

To keep streams composable and permit custom transformations, the source and bundler should remain independent logical steps.

Given this would be a breaking change, the release this would come out in should include #22

[Silic0nS0ldier]

This has been started with https://github.com/userfrosting/vinyl-fs-vpath