Support PNG as the page icon

[gabrieltempass]

Right now, if you use st.set_page_config() to set a PNG as the page icon, it seems that the function will convert the image to a JPEG (losing its transparent background properties):

streamlit/lib/streamlit/commands/page_config.py

Line 107 in 06330bd

output_format="JPEG",

So a PNG like this:

Will end up in the page icon like this:

It would be nice to have the option to set a PNG as the page icon and maintain its transparency.

[rbeauchemin]

I am also facing this, but found a way to fix it. I'll post a PR with the change to support transparency.