Update bclargs.py

> ./bclargs.py 
Namespace(background=False, render_anim=False)

> ./bclargs.py -b --re                                                                                                                                                                                          
Namespace(background=True, render_anim=True)                                                                                                                                                                    

> ./bclargs.py --help
usage: bclargs.py [-h] [--render-anim] [--background]

Parse command-line arguments passed to Blender

optional arguments:
  -h, --help         show this help message and exit
  --render-anim, -a  Render frames from start to end (inclusive)
  --background, -b   Run in background (often used for UI-less rendering)