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.

No, I don't think we need the pattern when there are scopes. Yes, I'd much rather have the one way of specifying this, but I don't want to leave people out in the cold either. I actually checked Grim for a way of deprecating configuration values :laughing: Maybe we could do it two-phased?

1. Convert scoped `column` values but leave the patterns in place. Issue a deprecation warning (somehow) for the patterns values.
1. Remove support for the patterns values.