Support switching puzzle files in game (Backend)

[will-snavely]

One idea:
Define the concept of a Puzzle directory, somewhere in the Spire game directory.
Puzzle files must be placed into this directory to be seen by the mod.
To implement this, we probably need:

1. Mod configuration setting to point to the location of the puzzle directory. Default location of mods/puzzlemod/puzzles, or something.
2. Minor tweaks to the puzzle loading logic to support this new idea.

[will-snavely]

Initial version done.