A more elaborate WordPress plugin to add FontAwesome icons via shortcode

fontawesome.php

<?php
/**
 * Plugin Name: FontAwesome
 * Version: 0.1.0
 */

// If this file is called directly, abort.
if ( ! defined( 'WPINC' ) ) {
	die;
}

add_action( 'plugins_loaded', 'FontAwesome' );

function FontAwesome() {
	if ( false === FontAwesomeClass::$instance ) {
		FontAwesomeClass::$instance = new FontAwesomeClass();
	}
	return FontAwesomeClass::$instance;
}

class FontAwesomeClass {
	public static $instance = false;
	public function __construct() {
		// Load FontAwesome assets from CDN.
		add_action( 'wp_enqueue_scripts', array( $this, 'assets' ) );
		// add `defer` attribute to FontAwesome script.
		add_filter( 'script_loader_tag',  array( $this, 'defer' ), 10, 2 );
		// output an icon via [icon] shortcode.
		add_shortcode( 'icon', array( $this, 'icon' ) );
	}
	public function icon( $args = array() ) {
		$defaults = array(
			'prefix'	 => 'fas',
			'name'		 => false,
			'class'		 => false,
			'size'		 => false,
			'fixed'		 => false,
			'spin'		 => false,
			'pulse'		 => false,
		);
		$args = wp_parse_args( $args, $defaults );
		$args['class'] = explode( ' ', $args['class'] );
		$args['class'][] = $args['prefix'];
		$args['class'][] = "fa-{$args['name']}";
		if ( false !== $args['size'] ) {
			$args['class'][] = "fa-{$args['size']}";
		}
		if ( false !== $args['fixed'] ) {
			$args['class'][] = "fa-fw";
		}
		if ( false !== $args['spin'] ) {
			$args['class'][] = "fa-spin";
		}
		if ( false !== $args['pulse'] ) {
			$args['class'][] = "fa-pulse";
		}
		$args['class'] = implode( ' ', $args['class'] );
		$args['class'] = esc_attr( $args['class'] );
		$icon = '<i class="' . $args['class'] . '"></i>';
		return $icon;
	}
	public function defer( $tag, $handle ) {
		if ( 'fontawesome' === $handle ) {
		    $tag = str_replace( ' src', ' defer src', $tag );
		}
		return $tag;
	}
	public function assets() {
		wp_register_script( 'fontawesome', 'https://use.fontawesome.com/releases/v5.0.8/js/all.js' );
		wp_enqueue_script( 'fontawesome' );
	}
}