Add support for scoped configuration

Applies to #17

This allows the wrap guide to respect scope-specific settings for `editor.preferredLineLength`. I haven't figured out a good way to disable the wrap-guide for specific scopes. Perhaps just `wrap-guide.enabled`, defaults to `true`?

/cc @kevinsawicki @maxbrunsfeld 

Thanks. I can split this out into a separate PR (or just push the single commit, it is pretty low-risk) if you want more time to hash out the scoped-config change.