ModLoader: Make callback on unload optional
ModLoader: Fully unload Ribbit-installed mods
ModManager: Check minToadyVersion when loading mods

Also with this commit, the loadMods function won't quit
with an error when one or more modIds fail to load.  Instead,
errors will be printed to the console and the function will
end successfully.
README: Add docs for mod metadata
Version 0.2.0.