Custom validation callback

[jhuliano]

Hello,
If found this package pretty useful but the "missing" criteria doesn't match with my needs.
So I would propose (as a new feature) having a custom, optional, callback as the last parameter so the user can modify this at his will.

It could look something like this (3 lines change):

function defaultsDeep(target, objects, isMissing) {
  target = target || {};
  isMissing = isMissing || ((val) => val == null);
  [...]
      if (isMissing(val)) {
        target[key] = value;
      }
  [...]
}