Add sub-command support

[tianyingchun]

[so1ve]

Clerc already supports subcommands now. Just use spaces

e.g.

defineCommand('parent children')

[tianyingchun]

but it should like command can use e.g. command git commit

1. help git --> should give nice command message (now give error, command not found)
2. help git commit ---> should give nice message for commit subcommand.