Write new commands into readline history

If I've entered a command, not just hit enter to repeat the last one, add it to the in-process readline history. Previously, readline history wasn't available until I'd quit and restart to load it all back in from `~/.rbx_debug`. `~/.rbx_debug` is still only written to after valid commands, but readline history includes whatever I happen to type in, so I can edit my stupid typos.