Provide correct info for function 'timeouts'

[abhirockzz]

trafficstars

CLI response for fn create fn --help and fn update fn --help returns incorrect info for timeout and idle_timeout e.g. AFAIK, the default idle_timeout is 30 seconds

--timeout value           Function timeout (eg. 30) (default: 0)
--idle-timeout value      Function idle timeout (eg. 30) (default: 0)

Please provide info about the maximum and minimum limits as well

[denismakogon]

Problem: https://github.com/fnproject/cli/blob/master/commands/init.go#L109. cli.IntFlag is problematic for us because we don't want to have a copy of Fn constraints in CLI, so, I'd prefer to use cli.StringFlag with some string-to-int conversion.