deferential: Promise/Callback Dual API Helper

deferential is a lightweight JavaScript utility designed to assist in building APIs that support both Node.js-style callbacks and native ES6 Promises. It provides a `Deferred` object, which acts as an intermediate for managing the resolution or rejection of a promise, and a `nodeify` method to conditionally return a promise or invoke a callback. The package is at version 1.0.0, indicating a stable but likely infrequent release cadence given its focused scope and the maturity of native Promise implementations. Its key differentiator lies in offering a minimalistic approach to deferred patterns, avoiding the heavier overhead of full-fledged promise libraries like Q or Bluebird by leveraging native browser/Node.js ES6 Promises. It's particularly useful for adapting existing callback-based code to also expose a promise-based interface without rewriting the core logic.