If the user runs a streamlit report via python script.py, they might be confused when nothing streamlit-ish happens.

(Joel, specifically, was confused by this: https://streamlit.slack.com/archives/CE2UZQX5M/p1568690189004600)

We should consider giving the user a heads up in this scenario:

When the streamlit lib is imported and we're not running within the streamlit command, display something like

Your streamlit app is running in "raw" mode, and isn't connected to a browser. Run
$ streamlit run [script.py]
to run your app within streamlit.

(To suppress this warning, you can add
[global]
warnOnRawRun = false

to your ~/.streamlit/config.toml file)