1.优化代码结构，抽象预览接口服务

2.新增更多图片预览格式支持

jodconverter-web/src/main/java/cn/keking/service/FilePreview.java

@@ -0,0 +1,11 @@
+package cn.keking.service;
+
+import org.springframework.ui.Model;
+
+/**
+ * Created by kl on 2018/1/17.
+ * Content :
+ */
+public interface FilePreview {
+    String filePreviewHandle(String url, Model model);
+}