fix(creator): FFCreatorCenter cannot customize the fileName

[electroluxcode]

fix https://github.com/tnfe/FFCreator/issues/326

在这个代码中

https://github.com/tnfe/FFCreator/blob/abbbb1104f484278cc61c3be65408f2a6c7cc3a7/lib/conf/conf.js#L48

给outputDir设置了一个默认值，导致了下面的代码每次都会进入 if (this.inCenter && outputDir) ，这导致了 output 这个变量永远都会被 uuid 设置，从而用户 实例化的 output永远不生效

https://github.com/tnfe/FFCreator/blob/abbbb1104f484278cc61c3be65408f2a6c7cc3a7/lib/creator.js#L99-L105