Config loader needs callable

[jb098]

Currently the config load expects a callable which seems to prevent it from loading in something as simple as a dictionary, which seems better.

I feel like we should be checking whether it's a callable or a dict as I think these are the two most common cases and loading in accordingly.

I'm looking at you for feedback on this one @aecay .

[aecay]

IMO, we should require it to be something that implements the Mapping interface. That would include dictionaries, but not callables. However, we only need the callable for default_config_loader (in config.py). We can rewrite that as a Mapping. (And in general, any hypothetical 3rd party callable could be so rewritten). Then we don't need to typecheck anything and can just use the methods provided by the Mapping interface.