Atmos should warn if not on latest version

[osterman]

what

• Emit a warning to STDERR that an out-of-date version of atmos is in use
• Support a atmos.yaml configuration setting to specify a list acceptable semver ranges
• Use the https://api.github.com/repos/cloudposse/atmos/releases API to determine the latest release satisfying semver ranges

why

Atmos is under active development. Bugs are constantly fixed. Older versions cause grief that could be avoided by upgrading.

example

versions:
- '~> 1.4'
- '! 1.4.2'

atmos version
1.4.1

atmos terraform plan...
WARNING: Latest version of atmos is 1.4.2; you're using atmos 1.4.1
...

[osterman]

[aknysh]

updating to the latest version breaks all the scrolling on the site.
We'll have to review all of that and prob add/update some CSS before updating.

(this is usually happens with Node, React and other JS frameworks - too many dependencies, too many people involved, they can't test everything, so in my experience, it almost always breaks one thing or another)

[osterman]

updating to the latest version breaks all the scrolling on the site.

To be clear, I'm not talking about docusarus, but atmos itself should emit a dialog like this if there's a newer version available.

[llpham14]

https://cloudposse.atlassian.net/browse/DEV-903