Preload polygon labels of output json file when user specify output path when startup

[chilin0525]

Hi,
I found Labelme do not collect polygon label when executing with specific output path(e.g. labelme -o label --autosave), so user may need recreate label or using --labels argument. I expecting when user specify output path, Labelme will collect label which already in output file and display in Polygon Labels of GUI.