Support custom Plausible script path

Add a PLAUSIBLE_SCRIPT config option so site-specific scripts
(js/pa-XXXXX.js) and legacy script extensions can be proxied. The
script is still served locally from /js/script.js and the cache key
is derived from the script URL so changing the script takes effect
immediately.

Closes #14

config/plausible-proxy.php

@@ -3,5 +3,17 @@
 // Config for Laravel Plausible Proxy
 
 return [
+    /*
+     * The Plausible instance to proxy, e.g. https://plausible.io or your
+     * self-hosted instance.
+     */
     'domain' => env('PLAUSIBLE_DOMAIN', 'https://plausible.io'),
+
+    /*
+     * Path of the script to fetch from the Plausible instance. Use this for
+     * site-specific scripts (e.g. js/pa-12345ABCDE.js) or legacy script
+     * extensions (e.g. js/script.outbound-links.js). The script is always
+     * served locally from /js/script.js.
+     */
+    'script' => env('PLAUSIBLE_SCRIPT', 'js/script.js'),
 ];