Add a method to customize the help message of a command

[vincent-lg]

Brief overview of PR changes/additions

This pull request adds a method 'get_help' to all commands. By default, this method simply returns the command's docstring, but one can create a command that still uses 'auto_help', but displays a customizable message.

Motivation for adding to Evennia

The 'auto_help' system is great, but it's either on or off. This pull request offers an alternative: a way to customize the help of commands without rejecting their docstring.

Example: I have a set of commands with sub-commands (separated by spaces), like "room" (master command), "room list" (see the list of rooms in the area), "room name" (change the name of the room) and so on. When one enters "help room", he/she should see the other sub-commands available. Updating the "room" docstring could increase the potential for errors. But the help system could auto-generate help for this command (and sub-commands) very easily.

[Griatch]

See the Evennia style guide here.

• self should not be included as an argument
• All arguments should list its type within parentheses. it comes to Evennia entities, I usually use Object rather than ObjectDB for clarity, so for example caller (Object or Player). The cmdset should have CmdSet as type.
• Since the return is not implicit, the Returns block should be listed, something like

  Returns:
      docstring (str): The help text to return for this command. 
  

[Griatch]

This looks like a good idea. See my note about the Evennia policy on docstring syntax.

[Griatch]

Merged with a rebase.