Updated class to use WP filters instead. 

Summary of changes and original pull request history [here](https://github.com/twittem/wp-bootstrap-navwalker/pull/129).

## tl;dr
It is more robust to rely on Wordpress' filter/hook framework than to subclass Walker_Nav_Menu. This avoids code duplication and arguably increases readability. Updated class is used and behaves exactly like the original class.