Add a public way of setting config values associated with a source

:warning: WIP :warning:

Previously, the private method `::addScopedSettings` was used to associate config settings with a source other than the user's config file. It bypassed schema validation.

This is being driven out by changes made in atom/snippets#86; to fix deprecation warnings required calling private methods on Config.
I think we need to clarify what we mean by source here. Maybe it's useful to say "associates the value with a given source, such as a path to a file on disk."