feat: add isNil for type checking

[alvis]

This PR provides a very simple, and yet essential, function for type checking, as
_.nil is not a primitive variable but an object. This function is particularly
useful in a consume handler which has a signature of consume<U>(f: (err: Error, x: R | Highland.Nil, push: (err: Error | null, value?: U | Highland.Nil) => void, next: () => void) => void): Stream<U>. Only with this function and an
updated typescript or flow definition, x can be distinguished between R and Highland.Nil.

Example:

source.consume(function (err, x, push, next) {
  if (err) {
    // pass errors along the stream and consume next value
    push(err);
    next();
  } else if (_.isNil(x)) { // type guard is only possible with this function
    // pass nil (end event) along the stream
    push(null, x);
  } else {
    // pass on the value only if the value passes the predicate
    if (f(x)) {
      push(null, x);
    }
    next();
  }
});

Note: This PR also comes with a simple test.

[vqvu]

Do you need this in the 2.x branch or the 3.x branch?

If you're going off the currently published documentation, that's the 2.x branch, so you'll want to rebase this onto caolan:2.x instead of caolan:master.

[alvis]

Thanks for pointing out. I'm using 2.x, so it should merge with the 2.x branch.

I've since changed the target branch.

[vqvu]

I've released this in 2.13.0

[alvis]

Thank you.