add argparse, cli, and show figures instead of saving them

[samsipe]

This addresses Issue #4

Thanks for open sourcing this project. Figured I could contribute a few things back on your project:

• added a new .gitignore
• removed hard dependency on Matplotlib 3.3.3 since it was unnecessary
• made main.py callable
• added argparse
• made satellite-constellation --test work in the command line
• updated docs

🍻