Use defer/async where possible

[php-coder]

Also we may use <script onload>.

Also this issue may be related to #221

See also:

• https://developer.mozilla.org/en-US/docs/Web/Guide/HTML/Tips_for_authoring_fast-loading_HTML_pages#Use_async_and_defer.2C_if_possible
• https://learn.javascript.ru/external-script (in Russian)
• http://www.growingwiththeweb.com/2014/02/async-vs-defer-attributes.html
• http://davidwalsh.name/html5-async
• https://flaviocopes.com/javascript-async-defer/
• https://www.html5rocks.com/en/tutorials/speed/script-loading/