Added notice for when a new version is available to upgrade to

[Xemdo]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

People are not always aware when a new version is available. This feature will notify the user (once a day) when the version of the Twitch CLI they're using is out of date, and give instructions on how to update.

Description of Changes:

• Added update notice after executing any command successfully
• Added ability to disable these notices
  • Automatically disabled in CI/CD environments, thanks to the CI env variable
  • Can be disabled by DISABLE_UPDATE_CHECKS in the config file, or TWITCH_DISABLE_UPDATE_CHECKS env variable
• Only executes once a day, which is checked by LAST_UPDATE_CHECK in the config file

Checklist

• [X] My code follows the Contribution Guide
• [X] I have self-reviewed the changes being requested
• [X] I have made comments on pieces of code that may be difficult to understand for other editors
• [X] I have updated the documentation (if applicable)