Added ability for CDN assets (#6)

* Added ability for CDN assets

* Update README.md

Author: TcFxGt4AGE

README.md

@@ -63,6 +63,13 @@ class Page extends SiteTree {
 ## Relative Assets Extension
  - Prefixes all references to `assets/*` with a preceeding slash i.e. they become something like `/assets/some.png`
  - If [zaininnari/html-minifier](https://github.com/zaininnari/html-minifier) is installed minifies the HTML output before rendering.
+ - If Assets.host config is set then Assets will be prefixed with this host.
+ 
+```yaml
+Assets:
+  host: '//example.com/'
+```
+Would equal `//example.com/assets/image.png`
 
 ## FooterMenuExtension
  - Adds a new *ShowInFooter* option to the page settings

code/SS_RelativeAssetsResponse.php

@@ -22,8 +22,10 @@ public function setBody($body)
     {
         $body = (string)$body;
 
+        $prefix = Config::inst()->get('Assets', 'host') ?: '/';
+
         $body = str_replace('"/assets/', '"assets/', $body);
-        $body = str_replace('"assets/', '"/assets/', $body);
+        $body = str_replace('"assets/', sprintf('"%sassets/', $prefix), $body);
 
         if (!json_decode($body) && class_exists('zz\Html\HTMLMinify')) {
             $this->body = zz\Html\HTMLMinify::minify($body, [