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 

The thing that it doesn't support is the file name matching feature. From: https://github.com/atom/wrap-guide/blob/master/docs/index.md

```coffee
'wrap-guide':
  columns: [
    { pattern: '\.mm$', column: 200 }
    { pattern: '\.cc$', column: 120 }
    { scope: 'source.gfm', column: -1 }
  ]
```

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?