Add module for a cleaner navigation walker

[retlehs]

this is the same walker from the sage theme except for:

• bootstrap-specifics have been removed
• make the walker the default if one isn't defined (basically reverting Remove Roots_Nav_Walker as default for all menu sage#1217)

[retlehs]

@vinkla thanks, but will tackle those outside of this PR.

[QWp6t]

plugin_dir_path is just a wrapper for dirname(__FILE__)

Could use require_once __DIR__ . '/lib/utils.php;

[vinkla]

@retlehs okay, great. Let me know if I can do anything to help.

Out of curiosity, what are the standards set for this package?

[vinkla]

What about naming the directory src instead of lib? src is much more common than lib.

Since nav-walker.php contains a class I think it would be good to show that in the file name. This is also common practice. I think we should name it NavWalker.php instead.

[swalkinshaw]

@vinkla coding standards? See here: https://github.com/roots/soil/blob/master/ruleset.xml and more details here https://github.com/roots/guidelines

[vinkla]

@swalkinshaw Yes, exactly what I was looking for. Thanks!