add filter for frontend dependencies

This filter makes it much easier to extend WPLF client side. 

Basically it allows users to include custom scripts before WPLF.
This is useful with Google reCAPTCHA, if you only want to include
the reCAPTCHA api.js on pages that have forms on them. 

Sky is the limit.

Author: k1sul1

classes/class-cpt-wplf-form.php

@@ -512,7 +512,13 @@ function maybe_enqueue_frontend_script() {
     global $post;
 
     // register the script, but only enqueue it if the current post contains a form in it
-    wp_register_script( 'wplf-form-js', plugins_url( 'assets/scripts/wplf-form.js', dirname(__FILE__) ), array(), WPLF_VERSION, true );
+    wp_register_script( 
+      'wplf-form-js', 
+      plugins_url( 'assets/scripts/wplf-form.js', dirname(__FILE__) ), 
+      apply_filters( 'wplf_frontend_script_dependencies', array() ),
+      WPLF_VERSION, 
+      true
+    );
 
     if( is_a( $post, 'WP_Post' ) && ( has_shortcode( $post->post_content, 'libre-form') || $post->post_type === 'wplf-form') ) {
       wp_enqueue_script( 'wplf-form-js' );