Print a message before running commands if we find a newer version of
Kit is available on GitHub. If the current version of Kit doesn't look
to be a regular release, or if any error occurs while checking, we don't
print anything (but do print some debug logs).

Add flag to version subcommand disable update notifications:

  kit version --show-update-notifications=false

Settings is 'saved' by touching a file in $KITOPS_HOME

Note, flag is attached to the 'version' subcommand as it seems there is
no convenient way to specify it on the root command; it cannot be
handled in PersistentPreRun and handling it in the root command's RunE
breaks the help text for 'kit' (specifically, the 'usage' field)