Add some support for the Lua API

[paulyoung]

These changes enable:

• Starting a game on PDSystemEvent::kEventInitLua to get access to the Lua runtime.
• Adding a function to the Lua runtime.
• Retrieving a string function argument within a function added to the Lua runtime.

There's a lot more to this API but this covers my use case for now, and hopefully makes it easier for others to extend if necessary.