Rework Navigation Menu: Builds off pull request #239 for issue #200

In this changeset:
 * Only set button and menu if container is not null.
 * Break up the "return early" conditional into multiple parts.
 * Shorten the name of the toggled class from `toggled-on` to `toggled`.
 * Restore the "Hide menu toggle button if menu is empty." documentation.
 * Remove the `nav-menu` class. The previous code would replace any custom custom classes applied via `wp_nav_menu()` or template file.
 * Replace the `nav-menu` selector is style.css with selectors that will target both a custom nav menu and a page menu. These changes could use critical feedback. Do you know of an edge case where they might not work as expected?