Consider supporting Webpack configurations that export a Promise

[pyrrho]

Webpack -- I believe since version 2 -- has been able to export a promise as it's configuration object. At the time of writing, filing-cabinet does check if the config object is a function, but this if branch will be spuriously matched in the case of an exported Promise, and will (eventually) result in result in an UnhandledPromiseRejectionWarning being thrown.

I'm not sure if there's an easy fix for this missing feature, though, so I would understand if handling promises in this manner isn't in scope for the project.