usage.php

<?php 

vip_register_async_script( 'twitter', 'https://platform.twitter.com/widgets.js' );

vip-async-scripts.php

<?php

function vip_register_async_script($slug, $url) {
  global $vip_async_scripts;

  $slug = sanitize_key( $slug );
  if ( ! isset( $vip_async_scripts[ $slug ] ) ) {
    $vip_async_scripts[ $slug ] = $url;
  }
}

add_action( 'wp_footer', function() {
  global $vip_async_scripts;

  $scripts_output = '';
  if ( ! empty( $vip_async_scripts ) ) {
    foreach ( $vip_scripts as $script ) {
      $scripts_output .= sprintf( '<script type="text/javascript" id="heavy-async-%1$s" src="%2$s" async></script>', esc_attr( $slug ), esc_url( $url ) );
    }
  }

  echo $scripts_output;
} );