Allow --help for rbt commands.

In order to print help for a command, the user would need to call
"rbt help <command-name>". We now also allow the "--help" option
both before or after the command name.

For example, both "rbt --help <command-name>" and
"rbt <command-name> --help" will now print help for the command.

Ran rbt commands with '--help' in various positions.
rbt --help
rbt --help status
rbt status --help
etc.