Add Config::isDefault(keyPath)

spec/config-spec.coffee

@@ -72,6 +72,17 @@ describe "Config", ->
       expect(atom.config.getDefault('foo.bar')).toEqual initialDefaultValue
       expect(atom.config.getDefault('foo.bar')).not.toBe initialDefaultValue
 
+  describe ".isDefault(keyPath)", ->
+    it "returns true when the value of the key path is its default value", ->
+      atom.config.setDefaults("foo", same: 1, changes: 1)
+      expect(atom.config.isDefault('foo.same')).toBe true
+      expect(atom.config.isDefault('foo.changes')).toBe true
+
+      atom.config.set('foo.same', 2)
+      atom.config.set('foo.changes', 3)
+      expect(atom.config.isDefault('foo.same')).toBe false
+      expect(atom.config.isDefault('foo.changes')).toBe false
+
   describe ".toggle(keyPath)", ->
     it "negates the boolean value of the current key path value", ->
       atom.config.set('foo.a', 1)

src/config.coffee

@@ -168,6 +168,13 @@ class Config
     defaultValue = _.valueForKeyPath(@defaultSettings, keyPath)
     _.deepClone(defaultValue)
 
+  # Public: Is the key path value its default value?
+  #
+  # Returns a {Boolean}, `true` if the current value is the default, `false`
+  # otherwise.
+  isDefault: (keyPath) ->
+    not _.valueForKeyPath(@settings, keyPath)?
+
   # Public: Push the value to the array at the key path.
   #
   # keyPath - The {String} key path.