Update Checking Improvements

[joshmedeski]

What would you like sesh to do?

As a user, I want sesh to let me know when there is a new upgrade available.

Feature ideas

• Use a tmux display-message when tmux is running?
• Use a tmux popup (custom session) displaying upgrade release notes to the user
• Create a type of 'changelog' command to show the user previous version
• Auto-detect installation method (ex: /opt/homebrew/bin/sesh means they can run brew update && brew upgrade sesh)
• Allow the user to configure how often they want to check for updates (hourly, daily, weekly?)
  • Determine a caching mechanism to store "last_checked" data