feature(downloader.py): Add HTTP proxy support

## General DiffusionBee may fail to download models when running in a proxy environment where: 1. ENV vars can not be evaluated due to Electron 2. Custom proxy settings may be required in general 3. A dedicated proxy should be used This PR adds a possibility to add a config file to DiffusionBee's dot dir (`.diffusionbee/config.ini`). By section, this may be extended for additional params in future. To define a proxy, just place: ``` [proxy] host: http://localhost:8000 ``` Why ini? While other files already are json I'd really like to prefer json but writing json is not beginner and human friendly. Therefore, I'd stick to ini. ## Issue Fixes #445 ## Ideas This could also be added to the GUI in future to read/write settings.