highland: add the definition for the new isNil() for type guarding

This commit add the definition for the a very simple, and yet essential, isNil
function for type checking. This function is particularly useful in a consume
handler.

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();
  }
});
```

See https://github.com/caolan/highland/pull/645

types/highland/index.d.ts

@@ -89,6 +89,17 @@ interface HighlandStatic {
 	// UTILS
 	// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 
+  /**
+	 * Returns true if `x` is the end of stream marker.
+	 *
+	 * @id isNil
+	 * @section Streams
+	 * @name _.isNil(x)
+	 * @param x - the object to test
+	 * @api public
+	 */
+  isNil<R>(x: R | Highland.Nil): x is Highland.Nil;
+
 	/**
 	 * Returns true if `x` is a Highland Stream.
 	 *