Check streamlit version on run

[tvst]

When the user types streamlit run foo.py, check whether a new version of Streamlit is available in PyPI and, if so, print the following message to the terminal:

  A new version of Streamlit is available. [in bold white]
  See what's new at https://streamlit.trydiscourse.com/c/announcements

  Enter the following command to upgrade:
  $ pip install streamlit --upgrade [in green]

Notes:

• Use the click package to print in color.
• Add a blank line at the end of the print block, so anything we print after that will be spaced out properly.

Implementation details

0. You can use this to check for updates: https://pypi.org/pypi/streamlit/json . More info.
1. We don't want to ping PyPI every time, so a simple solution would be to only look for updates with a 5% probability.
   Alternately, we could check for updates just once a day by using st.cache with persist to cache the update-checking function to disk, and add a max_age parameter to st.cache that lets you set an age at which the cache becomes invalid.
2. If the user is offline, don't show any errors.
3. A logicat place to put the update check is in cli.py inside _main_run().

[tconkling]

@tvst should the styling on this text be flipped? That is, should the "new version" notice at the top be green, and the command prompt be bold white? This would fit the pattern of, e.g., the

  You can now view your Streamlit report in your browser. (green)

  Local URL: http://localhost:3000 (bold white)
  Network URL: http://10.0.1.128:3000 (bold white)

notice.

[tconkling]

@tvst should the styling on this text be flipped? That is, should the "new version" notice at the top be green, and the command prompt be bold white? This would fit the pattern of, e.g., the

  You can now view your Streamlit report in your browser. (green)

  Local URL: http://localhost:3000 (bold white)
  Network URL: http://10.0.1.128:3000 (bold white)

notice.

(Yes)