bug when tagging choices in a thread

if i tag one choice, all the choices i'm threading, even if it's separate threads, get the tag. irritating when you're using tags for CSS styling purposes.

As in:
```
=== room ===
- (opts)
<- room(-> room.act, -> opts)
<- dialogue(-> room.talk, -> opts)
<- exits

= act
* [a choice]

= talk
* [a choice]

= exits
#.exit
+ [EXITS]
#.exit__submenu
+ + [ANOTHER ROOM]
-> next_room
```
even though i only meant to tag `[EXITS]` and `[ANOTHER ROOM]`, for some reason all the choices i am gathering get tagged with `#.exit`