Added --share option so that it can also be used with Google Colab

[shinshin86]

Thank you for cool project!

I've added the --share option for Gradio to make this project accessible from Google Colab as well.

Due to Google Colab's dependencies, errors do appear on the Colab console when starting up, but I have confirmed that it can be used without any issues.

I would appreciate it if you could review this Pull Request.

Thank you!