Return subcommand if its already registered

[bradgessler]

When running Thor in a development environment on a server a code reloader, I noticed the subcommand would register duplicates like this:

tinyzap/server [cli-puma] → bin/tinyzap
Commands:
  tinyzap echo                                                          # Echos the input from stdin
  tinyzap help [COMMAND]                                                # Describe available commands or one specific command
  tinyzap links                                                         # Manage links
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap links list                                                    # Lists links
  tinyzap login                                                         # Login to TinyZap
  tinyzap whoami                                                        # Prints your current login information

tinyzap/server [cli-puma] →

This PR fixes that problem by checking to see if the command is registered already. If it is, it returns the existing subcommand.

tinyzap/server [cli-puma] → bin/tinyzap
Commands:
  tinyzap echo                                                          # Echos the input from stdin
  tinyzap help [COMMAND]                                                # Describe available commands or one specific command
  tinyzap links                                                         # Manage links
  tinyzap links create -d, --description=DESCRIPTION -t, --title=TITLE  # Create a new link
  tinyzap links help [COMMAND]                                          # Describe subcommands or one specific subcommand
  tinyzap links list                                                    # Lists links
  tinyzap login                                                         # Login to TinyZap
  tinyzap whoami                                                        # Prints your current login information

[rafaelfranca]

Do we need to return the command class? It seems we only need to not register the command again. But what happens if people wants to override the subcommand to have different code? Let's say a library trying to enhance an already existing class. It seems that would break that behavior.