Add Optional.or()

Currently there is no simple way to execute a supplier if current Optional is empty,
that can be seen as the the dual of flatMap() that executes a function if the current Optional is not empty.

A possible workaround is to use
  map(Optional::of).orElseGet(supplier)
which while working create an unnecessary Optional and is less readable.

The proposed implementation (by Paul Sandoz):
public Optional<T> or(Supplier<Optional<T>> mapper) {
      Objects.requireNonNull(mapper);
      if (isPresent()) {
          return this;
      } else {
          return Objects.requireNonNull(mapper.get());
      }
  }

For more information, here is the relevant discussion on core-dev:
http://mail.openjdk.java.net/pipermail/core-libs-dev/2015-April/032904.html