This is a follow up to a previous PR. The async attribute can cause issues with some js files. For instance I added the lozad.js (https://github.com/ApoorvSaxena/lozad.js) for lazy loading and included it via my theme. However, with the current automatic inclusion of "async" things didn't work.
So this update allows Theme developers to prevent the async attribute by passing in "null".
Example: ```<?php echo Theme::javascript('js/lozad.min.js', DOMAIN_THEME, null); ?>```
If the Theme developer doesn't include "null" the default will be to use "async".
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