Move options to separate `config.py` file

That way, modifying or updating `gh-issues-import.py` won't affect
the password, and won't publish the password for all to see in public repos.

Code taken from https://github.com/dliuzzi/tools/commit/5bbfe04775d83343268af35dd6b0a552a0ee43b4
created as a new commit, rather than a cherry pick or merge to avoid merge conflicts.
Issues are now exported chronologically

Original commit by 'dliuzzi' (http://github.com/dliuzzi)
in https://github.com/dliuzzi/tools/commit/5d190cdc994240379d96df2206fcc218cabb09a9