Add Windows and Linux support

Desktop Wallpaper Changer fetches images from popular sources and set those as your desktop wallpaper. Currently supported sources are:

• NASA Astronomical Picture of the Day
• Bing Picture of the Day
• Random Pictures from Unsplash.com
• National Geographic Picture of the Day
• Random Images from Desktoppr.

Initial Test Structure and CLI Improvements

Updated

• Do not search in options mode for unsplash

Added

• Added initial test structure