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.
Man that's really tricky. I like the idea of reverting to the value at the time set was called rather than the official default.