Add placeholder passage for first real battle

Since mechanics are being worked out separately, there's a temporary
passage connecting pre-battle to post-battle story.
Add post-battle passage to intro

The introduction has the Player meet Kerim, Marianela, and Mem, and ends
when the Player successfully finishes off the shades at the back of
Mem's house. All passages in the intro are tagged as such.
Merge branch 'develop' of https://github.com/amclark42/blue-period-game into fullbattle
Add battleController

The passage "battleController" ensures that combatants take their
turns according to initiative order, and serves as a shell for
Player actions/responses during battle.