Implement self-updating

[maouw]

It would be desirable for hyakvnc to:

• check for any updates on this repository when it is run unless it is being silenced (add a -q flag?)
• if there is an update available:
  • print a message
  • if running interactively and there is not an environment variable like HYAKVNC_PROMPT_ON_UPDATE=1:
    • prompt to update
      • if yes, install update and restart

This could be implemented by changing the one-line install scriptlet to git clone the repository into, e.g., ~/.hyakvnc/hyakvnc and soft link ~/.local/bin/hyakvnc to '~/.hyakvnc/hyakvnc, then use the necessary git` command that checks for updates on the remote.