This pr does three things: 1. add `delete_existing_files` property for outfile/export. If `delete_existing_files = true`, export/outfile will delete all files under file_path first. 2. add p2 test for export 3. modify docs
This pr does three things: 1. add `delete_existing_files` property for outfile/export. If `delete_existing_files = true`, export/outfile will delete all files under file_path first. 2. add p2 test for export 3. modify docs