Default or provide an easy way to show full help when a command is run with no arguments

[lgarron]

It is common for programs to print either full help or at least an abbreviated help if no arguments are provided. (For example, `clap does this.)

If there are any required arguments, cmd-ts prints an error and then suggests passing --help. It would be more user-friendly to show the full help immediately if no arguments were passed to the command at all, even if that behaviour is opt-in.