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 still want to support this? I would be inclined to take it out. We could have a bit of code that reads it, and converts it to scoped properties. Does it do something different than the scoped enable + scoped preferred line length? If so, can we add that functionality without the non-standard config?