Adds --default-path, if specified, into LOAD_PATH

See: https://github.com/rspec/rspec-core/issues/830

This small patch adds a specified default-path to the list of libraries to load, so that specs which `require 'spec_helper'` will not fail when a default-path option is specified.

Because `--default-path` can be either a file or a directory, the path is only added when it is a directory.