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

README.md

@@ -36,9 +36,12 @@ The `config/plausible-proxy.php` file allows you to customize the Plausible doma
 ```php
 return [
     'domain' => env('PLAUSIBLE_DOMAIN', 'https://plausible.io'),
+    'script' => env('PLAUSIBLE_SCRIPT', 'js/script.js'),
 ];
 ```
 
+`PLAUSIBLE_SCRIPT` is the path of the script on your Plausible instance. Whatever script you choose, it is always served locally from `/js/script.js`.
+
 ## Usage
 
 ### 1. Proxying the JavaScript file
@@ -53,6 +56,32 @@ After installation, you can include the script like this:
 
 The route `/js/script.js` is handled automatically and caches the script for 6 hours.
 
+#### Site-specific script (pa-XXXXX.js)
+
+Plausible's current installation instructions use a site-specific script such as `https://plausible.io/js/pa-12345ABCDE.js`, where optional measurements (outbound links, file downloads, etc.) are configured in the Plausible dashboard. To use it, set the script path in your `.env`:
+
+```env
+PLAUSIBLE_SCRIPT=js/pa-12345ABCDE.js
+```
+
+Then load it locally and tell it to send events through the proxy with the `endpoint` option:
+
+```html
+<script async src="/js/script.js"></script>
+<script>
+  window.plausible=window.plausible||function(){(plausible.q=plausible.q||[]).push(arguments)},plausible.init=plausible.init||function(i){plausible.o=i||{}};
+  plausible.init({ endpoint: '/api/event' })
+</script>
+```
+
+#### Legacy script extensions
+
+Legacy script variants work the same way, for example:
+
+```env
+PLAUSIBLE_SCRIPT=js/script.outbound-links.js
+```
+
 ### 2. Sending events to Plausible
 
 The script loaded from `/js/script.js` sends events to `/api/event` on your own domain, which forwards them to Plausible. This helps prevent blocking by ad blockers.