Run lazy-load script on DOMContentLoaded when necessary, to improve compatibility with plugins like Autoptimize

[felixarntz]

Summary

This PR can be summarized in the following changelog entry:

• Run lazy-load script on DOMContentLoaded when necessary to improve compatibility with plugins like Autoptimize.

Fixes #6

Checklist:

• [x] My code is tested.
• [x] My code is backward-compatible with WordPress 4.7 and PHP 7.0.
• [x] My code follows the WordPress coding standards.
• [x] My code has proper inline documentation.