add captcha settings

example/config.json

@@ -125,6 +125,17 @@
         "index":
         {
             "enabled":true
+        },
+        "captcha":
+        {
+            "length":5,
+            "phrase":"0123456789abcdef",
+            "background":
+            {
+                "r":46,
+                "g":52,
+                "b":54
+            }
         }
     },
     "cli":

src/webui/explore.php

@@ -215,8 +215,17 @@ if ($config->webui->index->enabled)
     );
   }
 
-  $captcha = new \Gregwar\Captcha\CaptchaBuilder();
+  $captcha = new \Gregwar\Captcha\CaptchaBuilder(
-  $captcha->setBackgroundColor(46, 52, 54);
+    $config->webui->captcha->length,
+    $config->webui->captcha->phrase
+  );
+
+  $captcha->setBackgroundColor(
+    $config->webui->captcha->background->r,
+    $config->webui->captcha->background->g,
+    $config->webui->captcha->background->b
+  );
+
   $captcha->build();
 
   $_SESSION['captcha'] = $captcha->getPhrase();