certain subcommands don't have a dedicated help message

[Dieterbe]

dtask help show-open shows a generic help message covering all commands and task show-open help executes show-open without showing any help message. seems to be hit or miss. e.g. for task help done it does show the help text for the done subcommand.

[naggie]

Ah, definitely a bug then -- show-open needs a help message. As for the order of the arguments, help must always precede the command. show-open help should search for open tasks containing the string help.

We should add a unit test to check all commands have associated help commands too I guess.