🎈 Width customizer for Streamlit Apps

As of now, you can only change your Streamlit app's layout width from narrow (default) to wide. You can switch to the wide mode persistently by adding this code at the top of your streamlit_app.py file:

# Use the full page instead of a narrow central column
st.set_page_config(layout="wide")

1. Now, wouldn't it be nice to have custom layout widths?
2. Wouldn't it be cool to be able to change these custom widths on the fly?
3. Wouldn't it be even sweeter to change your app's columns' width at a click of a button?

I've made an app for that!

Still barebones. I'm planning to add to it in the future.

Feedback and PR appreciated, as always :)