Makefile, main.cpp: allow external main

Allow an external `int main()` coming from outside this Git repo.

- Modify the Makefile to allow arbitrary objects to be passed in for
  linking via WORLDOBJECTS variable.

- Modify main.cpp to change `int main` to `int TsunagariMain` that can
  be called from an extern main.
.gitignore: add src/*/*.o