Add default configuration for images (#1979)

This patch allows configuring image processing instructions
for all images.
A use case is generating a fixed set of sizes (derivatives) for
all images, without having to specify them in the content markdown.

Example configuration in user/config/system.yaml:

images:
  defaults:
    derivatives: "[300,600,1200]"

system/src/Grav/Common/Helpers/Excerpts.php

@@ -294,6 +294,15 @@ class Excerpts
         if (Grav::instance()['config']->get('system.images.auto_fix_orientation')) {
             $actions[] = ['method' => 'fixOrientation', 'params' => ''];
         }
+        $defaults = Grav::instance()['config']->get('system.images.defaults');
+        if (is_array($defaults) && count($defaults)) {
+            foreach ($defaults as $method => $params) {
+                $actions[] = [
+                    'method' => $method,
+                    'params' => $params,
+                ];
+            }
+        }
 
         // loop through actions for the image and call them
         foreach ($actions as $action) {