don't load all the tabs when starting up, only the current one. Defer the loading of the others when switching to them.

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