Refactor: Improve REPL input command parsing

[jschuur]

Is there a third party library that would easily parse commands from the REPL and automatically create help text? Something like commander that I can use to pipe in anything that starts with a dot?

Could I just use commander? (see discussion here and here)

Otherwise, go for a simple way to list arguments to a command in the help text:

• Optional fourth parameter in CommandList becomes help text first column? E.g. models [update].

• Or (better idea): the index is models [update] and we always split off the first word for command parsing.

• [x] when implemented, change longer commands to use subcommands (e.g. .conversationlength becomes conversation length.

[jschuur]

Didn't go the commander route, but just modified the .help listing to show optional subcommands.

Completed for 0.6.0