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

* 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

split the logic into two functions: one to render the echoarea and one to rendere the minibuffer (when it's active).