Add NPCs to party & remove them

[mpirnat]

Sometimes one or more NPCs might join the party for a while. If we don't want to assume that they just hang back and never participate in combat, we should work out a way to add them to the party and remove them when they're done.

This entails modeling them somehow in TOML and object representation (like other forms of combatant), figuring out how to organize them on disk to load, loading them, and removing them.

[mpirnat]

Adding sidekicks in #63 chips away at this a little more... Yay!

Between that and being more careful about what we do with leftover "monsters" from an encounter, it's possible that this is mostly done. About the only thing we're not doing is shifting monsters over into the characters dictionary and serializing/deserializing them during saves. (Which is probably the trickier and more tedious part.)