--files_out command line argument

[petaflot]

Describe the limitation

everytime pdfarranger is started, files_out is reset

To Reproduce

Steps to reproduce the behavior:

1. cd to /some_work_dir
2. pdfarranger /some_source_dir/file0.pdf
3. select some pages, hit Ctrl+E
4. SELECT EXPORT DIRECTORY, ie /output_dir, hit enter
5. select other pages, hit Ctrl+E
6. export directory is remembered from last export
7. export done : close pdfarranger
8. pdfarranger /some_source_dir/file1.pdf
9. select some pages, hit Ctrl+E
10. SELECT EXPORT DIRECTORY, ie /output_dir, hit enter
11. etc.

problem: it is easy to forget to set export directory on the first export

Expected behavior

1. cd to /some_work_dir
2. pdfarranger --out_files=/export_dir /some_source_dir/file0.pdf
3. select some pages, hit Ctrl+E
4. export directory is already set to /export_dir