🚸 Use appropriate completions when ending combat

Using the prompt session to prompt for how to dispose of leftover
monsters at the end of combat meant we were getting the primary command
completion which was inappropriate for the situation, so switched it to
use a situationally-appropriate WordCompleter instead.

Again sort of addresses #29.

dndme/commands/end_combat.py

@@ -1,4 +1,5 @@
 import math
+from prompt_toolkit.completion import WordCompleter
 from dndme.commands import Command
 from dndme.commands.remove_combatant import RemoveCombatant
 from dndme.commands.show import Show
@@ -29,10 +30,12 @@ def do_command(self, *args):
 
         # Allow some leftover monsters to remain in the combat group;
         # perhaps some are friendly NPCs along for the ride?
+        choices = WordCompleter(['keep', 'remove', 'stash'])
         for monster in list(combat.monsters.values()):
             choice = self.session.prompt(
                 f"What should we do with {monster.name}? "
-                "[R]emove [S]tash [K]eep (default: Keep) "
+                "[R]emove [S]tash [K]eep (default: Keep) ",
+                completer=choices
             ).lower()
             if choice in ('r', 'remove'):
                 RemoveCombatant.do_command(self, monster.name)