the new `meta.data' field will be useful for holding completions data.

"no prev item" (or next) can be misleading during a completing-read

* cmd.c (cmd_previous_completion): add previous-completion (cmd_next_completion): add next-completion (cmd_insert_current_candidate): add insert-current-candidate * minibuffer.c (enter_minibuffer): support completions in minibuffer

buffer->top_line could end up being NULL if we blindly TAILQ_PREV...