Making gradio private by default, and adding --public argument

[MeijiIshinIsLame]

This may be more of a matter of opinion, so feel free to reject this if it doesn't fit.

Basically, I noticed that when I run edit_app.py gradio automatically runs a public link accessible from the inernet, and the only way to turn it off was to edit the code. I thought this could potentially be insecure to have it as the default behavior, since there are a lot of people using this who may not understand the implications.

I made the default behavior of edit_app.py gradio to be a private link, and I added a --public arg in case anyone wants to make their app accessible from anywhere.

Thank you.