Subcommands should show help with no args

[chrisdone]

This subcommand currently isn't too helpful:

chris@retina:~/Work/stack$ stack ide
Usage: stack ide [--help] COMMAND
  IDE-specific commands

Running stack by itself produces stack --help, but seems that subcommands still have this issue.

[HuwCampbell]

Can now use showHelpOnEmpty.

[mpilgrem]

I will propose a pull request that I think fixes this.