MINOR: cli: make "help" support a command in argument

MINOR: cli: make "help" support a command in argument

With ~100 commands on the CLI, it's particularly difficult to find a
specific one in the "help" output. The function used to display the
help already supports filtering on certain commands, so in the end it's
just needed to pass the argument of the help command to enable the
automatic filtering. That's what this patch does so that "help clear"
only lists commands starting with "clear" and that "help map" lists
commands containing "map" in them.