Added a way to set a custom user folder

[Leidtier]

Added a custom_user_folder.ini that can be used to set another path for the user folder than the default one.

@art-from-the-machine The location of the custom_user_folder.ini is right next to the main.py/Mantella.exe. If you prefer another location for this ini, you will have to adjust the path in setup.py