Play nice with GIFs

[nogarcia]

Describe the bug
When downloading an animated comic, the script downloads the bytes and names them as a PNG. The file fails to load in an image editor. 1264 is a still image in ImageMagick. The file is verifiably a GIF when viewed in a hex editor.

To Reproduce
Steps to reproduce the behavior:
1.

$ xkcdn --num 1264 # any animated comic

2. Open the file in EOG or any other image viewer.

Expected behavior
The image is recognized as a GIF and is aptly named.

Desktop (please complete the following information):

• OS: Ubuntu 18.04.1 LTS
• Version: latest

[nogarcia]

I converted the file in bytes to a string. The first five characters are "b'GIF".