<!-- Before you proceed: 1. Searched existing issues to avoid creating duplicates. 2. Searched http://support.typora.io/ since it may be already implemented. --> 打开多个文档的时候不新建窗口，在当前窗口新建tab页，这样多文档编辑的时候方便切换，或者可以选择新建窗口还是新建tab页
