Reform configuration loading in main.py

[richteer]

The config.json is opened in more than one spot in main.py, including one case where a Halibot class is instantiated just to call _load_config().

This should all be factored out into a load_config() function in main.py, and _load_config() on the Halibot class should take in a dictionary as an argument instead.

Writes to config.json in main.py should stay as they are, probably.

There should be some kind of more-centralized configuration loading system, so main.py and Halibot use the same interface for loading/writing a config file.

[sjrct]

Probably should have Halibot class manage config loading/writing. That way it can keep it up-to-date automatically for interactively adding/removing modules and agents. Perhaps automatically write the config out on exit.

[richteer]

Should we close this? I think we have address this already, plus PR #114 starts to go nuts with avoiding multiple config loads.