Unmanifested Asset Helper

[robwise]

When using something like the DLL plugin, we are using a completely separate config to do that. Therefore, this file is not in the manifest, since we cannot share manifests between multiple webpack configs.

That's okay for the moment because my team only uses the DLL plugin in development, so we can make sure to not hash the DLL plugin, and then we don't require reading from the manifest. However, we still need to properly grab the files from the correct webpack directory.

I am going to add two more methods to the API:

• unmanifested_stylesheet_pack_tag
• unmanifested_javascript_pack_tag

[justin808]

@robwise check out #20.