list `clean` command in help
auto merge of #169 : cburgdorf/cargo/name_clean, r=alexcrichton

There are other unnamed commands like `verify-project` but I'm unsure if they are **not** listed on purpose because they should be used directly by the user less frequently. However, the clean command seems like a command that should be listed here.

Not sure about the wording though. It could be more abstract like *remove build artifacts* but given it's current behavior I think *remove target directory* is somehow straight forward, too.