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 

Do we need the pattern when there are scopes? I guess it makes for difficult upgrading. 

It would just be nice to have only one way to specify config for this. 

> I thought about that, but I figured that it should support the scoped preferredLineLength value anyway ... and once you go that far, does adding another setting really add any value?

I would totally rather just use `preferredLineLength`.