A liquid tag to include a file from its path relate to Jekyll's source folder. Why? Because Jekyll's built-in include tag does not support including files outside of _includes folder.
Syntax: {% include_absolute path %}
Copy include_absolute.rb into /_plugins
js/common.js
{% include_absolute js/jquery.js %}
{% include_absolute js/bootstrap.js %}
css/common.css
{% include_absolute css/bootstrap.css %}
{% include_absolute css/ionicons.css %}
MIT