support db select option

(pull request Jimdo#96 from original repo)

Author: ihomyak

src/Prometheus/Storage/Redis.php

@@ -41,6 +41,9 @@ public function __construct(array $options = array())
         if (!isset(self::$defaultOptions['password'])) {
             self::$defaultOptions['password'] = null;
         }
+        if (!isset(self::$defaultOptions['db'])) {
+            self::$defaultOptions['db'] = false;
+        }
 
         $this->options = array_merge(self::$defaultOptions, $options);
         $this->redis = new \Redis();
@@ -97,6 +100,9 @@ private function openConnection()
             if ($this->options['password']) {
                 $this->redis->auth($this->options['password']);
             }
+            if($this->options['db']){
+                $this->redis->select($this->options['db']);
+            }
             $this->redis->setOption(\Redis::OPT_READ_TIMEOUT, $this->options['read_timeout']);
         } catch (\RedisException $e) {
             throw new StorageException("Can't connect to Redis server", 0, $e);