Add Save Options Module

[Justin99x]

Module that enables saving arbitrary json-encodable data directly to the character save file. Original issue here:
#1

The module extends mods_base.ValueOption and its subclasses by adding a set of SaveOption classes. The classes behave almost exactly like their parents, except their values are saved to the save file instead of to the mod's settings file.

Some changes to note since apple's informal code review a few weeks ago:

• Updated mods_base submodule to latest commit that includes _to_json() and _from_json() methods for all options.
• Added an optional on_load callback that that runs as a post hook to LoadPlayerSaveGame. This allows for applying any custom save data to the character immediately after the regular save data has been applied.
• GroupedOption and NestedOption are handled by checking if every child is either Grouped/Nested or a Save Option. If so, it can be registered as a save option. The new _(to|from)_json() methods are used to convert the values to JSON.

[apple1417]

Looks like most of the errors, bar the one spelling, are unrelated, and just broke due to newer releases. I'll get a fix in for them soon-ish.

[apple1417]

Please merge in/rebase on top of current master, which should fix the unrelated errors. Will merge when all checks pass.