Finalize CivOne API/plugins for 0.1.0-alpha.1

[Solen1985]

For alpha.1, the following API functions will be exposed to plugin makers.

• In game menus (modify text) (DONE)
• Civilizations (DONE)
• Civilization leaders (DONE)
• Modify units (DONE)
• Modify advances

Possibly, otherwise move to future release:

• AI movement
• Tribal hut outcome
• Modify buildings
• Modify wonders
• Game save data

Documentation and samples will need to be provided on the https://github.com/SWY1985/CivOne-Plugins repository.