Make ConfigObserver a proper mixin and export it

exports/atom.coffee

@@ -8,6 +8,7 @@ module.exports =
   File: require '../src/file'
   fs: require 'fs-plus'
   Git: require '../src/git'
+  ConfigObserver: require '../src/config-observer'
   Point: Point
   Range: Range
 

src/config-observer.coffee

@@ -1,4 +1,7 @@
+Mixin = require 'mixto'
+
 module.exports =
+class ConfigObserver extends Mixin
   observeConfig: (keyPath, args...) ->
     @configSubscriptions ?= {}
     @configSubscriptions[keyPath] = atom.config.observe(keyPath, args...)

src/display-buffer.coffee

@@ -15,7 +15,7 @@ ConfigObserver = require './config-observer'
 module.exports =
 class DisplayBuffer extends Model
   Serializable.includeInto(this)
-  _.extend @prototype, ConfigObserver
+  ConfigObserver.includeInto(this)
 
   @properties
     softWrap: null

src/space-pen-extensions.coffee

@@ -1,9 +1,9 @@
 _ = require 'underscore-plus'
 spacePen = require 'space-pen'
-ConfigObserver = require './config-observer'
 {Subscriber} = require 'emissary'
+ConfigObserver = require './config-observer'
 
-_.extend spacePen.View.prototype, ConfigObserver
+ConfigObserver.includeInto(spacePen.View)
 Subscriber.includeInto(spacePen.View)
 
 jQuery = spacePen.jQuery