Add $config['title_image'] in doc and use it also for login screen

.gitignore

@@ -17,6 +17,7 @@
 Thumbs.db
 config.php
 html/css/custom/*
+html/images/custom/*
 html/plugins/*
 !html/plugins/Test/
 includes/custom/*

doc/Support/Configuration.md

@@ -132,6 +132,11 @@ You can override a large number of visual elements by creating your own css styl
 `html/css/custom` so they will be ignored by auto updates. You can specify as many css files as you like, the order they are within your config 
 will be the order they are loaded in the browser.
 
+```php
+$config['title_image'] = "images/custom/yourlogo.png";
+```
+You can override the default logo with yours, place any custom images files into `html/images/custom` so they will be ignored by auto updates.
+
 ```php
 $config['page_refresh']     = "300";
 ```

html/pages/logon.inc.php

@@ -8,7 +8,13 @@
           <div class="panel panel-default">
             <div class="panel-heading">
               <h3 class="panel-title">
-                <img src="images/librenms_logo_light.svg">
+                <?php
+                if ($config['title_image']) {
+                    echo '<img src="' . $config['title_image'] . '">';
+                } else {
+                    echo '<img src="images/librenms_logo_light.svg">';
+                }
+                ?>
               </h3>
             </div>
             <div class="panel-body">