check both file and filename

README says to use filename property, but it only checks file property.
To maintain backwards compatibility it should check both.

Author: cesarandreu

index.js

@@ -20,7 +20,7 @@ module.exports = function(opts){
                 },
                 {
                     type: 'file', 
-                    filename: opts.file, 
+                    filename: opts.file || opts.filename, 
                     maxLogSize: opts.size || 10*1024*1024,
                     backups: opts.backups || 4,
                     category: loggerName
@@ -45,4 +45,4 @@ module.exports = function(opts){
 // add more info
 // 解析配置选项, 创建file logger, 支持log 内容配置
 // 从其他middleware中可以获取到 logger  
-// 日志格式可参看 log4js的 connetct-logger
+// 日志格式可参看 log4js的 connetct-logger