Consider switching to a JSON config

Currently the config is a JavaScript file, meaning it can literally run code. That's probably a bad idea. A better one would be to switch to JSON config.

Related to #30 (closed).

Pros of moving to JSON-based config:

• non-executable config file, yay!
• clear path to updating the config via LibResilient when original domain is not available

Cons of moving to JSON-based config:

• additional work, potentially substantial
• loading the config needs to be implemented explicitly (instead of relying on importScripts())
• cannot rely on updateViaCache() to update the config (which admittedly would not solve the update-during-disruption problem)