Adding setters for webhookUrl in Slack

Author: ashwanthkumar

src/main/java/in/ashwanthkumar/slack/webhook/Slack.java

@@ -96,6 +96,16 @@ public Slack parse(String parse) {
         return this;
     }
 
+    /**
+     * Update the webhook url of the underlying Slack service
+     * @param webhookUrl
+     * @return
+     */
+    public Slack setWebhookUrl(String webhookUrl) {
+        this.webhookUrl = webhookUrl;
+        return this;
+    }
+
     /**
      * Publishes messages to Slack Webhook
      *