Commands API (WIP)

[hkzorman]

Fixes #36

This is WIP, opened PR so people can start reviewing this change. These are tasks expected to be completed in this PR:

• [x] Execution context for NPCs
• [x] Implement helper functions to work with expressions
• [x] Implement variable set/read commands
• [ ] if/else command
• [ ] loop command
• [ ] query command
• [ ] Inventory commands
• [ ] Attend to https://github.com/hkzorman/advanced_npc/issues/22#issuecomment-338216849
• [ ] Registerable scripts
• [ ] do_punch() and do_rightclick()
• [ ] Implement logic for execution of commands
• [ ] Implement logic for execution of scripts
• [ ] Implement state script functionality

[hkzorman]

This branch followed a now obsolete approach. WiIl re-open pull request with newer code soon.