Add image as background

[lennart-rth]

Add an option to add a background image to the composition. For example a faint night sky with very dim stars.

1. The image file should be passed as a absolute path in the config.yml file in the settings Section.
   Maybe rename bg-color to bgand make it possible to parse color hex value or image path.
   If not specified the background should still be filled with black.
   This should be done in image_parser.py line 22, where the black background is currently added.
2. Currently the loaded satellite images are square in size. To blend in behind the background they must be clipped to a round shape.
   Maybe add a alpha layer mask with pillow to crop the black areas out. Luckily the Planet is always in the same position, but does this work for different zoom Levels too.