jp (shell): shell command (REPL mode), first draft: This command launch jp in REPL mode, allowing do normal jp commands with some facilities. Command can be selected with "cmd" (e.g. "cmd blog"). An argument can be fixed with "use" (e.g. "use output fancy"). Command is launched with "do", or directly with its name if it doesn't conflict with a shell command. Arguments completion is still TODO (only shell commands are completed so far).