function interceptPromise(prom, cb) {
  prom.then(cb, cb)
  return prom
}
Executes the given callback when the promise is finalized, whether it is
resolved or rejected.

@memberof Shim.prototype
@param {Promise} prom  - Some kind of promise. Must have a `then` method.
@param {Function} cb   - A function to call when the promise resolves.
@returns {Promise} A new promise to replace the original one.