JSON files unsupported as mainConfigFile

[bregenspan]

The error emitted when a mainConfigFile cannot be parsed implies support for JSON config files:

The config in mainConfigFile [path to file] cannot be used because it cannot be evaluated correctly while running in the optimizer. Try only using a config that is also valid JSON.

However, the config file must actually be valid JS that includes the JSON as a an object that is passed to requirejs.config() or similar, and not actually raw JSON. Is support for raw JSON something that would be considered for inclusion (if so, I'll submit a PR)?