Support GitHub url of main script

[snehankekre]

📚 Context

@arnaudmiribel you had mentioned that you'd someday like to support st {template}, where template is a GitHub repo. This PR makes incremental progress towards that vision.

🧠 Description of changes

• Supports st {template}, where template is the GitHub (.py) url of the main script. E.g.

  st https://github.com/streamlit/streamlit-example/blob/master/streamlit_app.py
  

• Creates a virtualenv with venv

TODO

• Refactor into functions to more conveniently reuse code
• Support Conda, Poetry

[arnaudmiribel]

Ohhh that's very cool! Thanks for jumping into this, I think starting from template repositories is a much better way to approach the problem.

Approving your PR, although I will add some stuffs in a subsequent chain of commits