Calling .extend multiple times does not properly deep merge

[dbanksdesign]

Test case:

var StyleDictionary1 = StyleDictionary.extend({
  foo: 'bar'
});
var StyleDictionary2 = StyleDictionary1.extend({
  foo: 'baz'
});
// StyleDictionary2.foo should be 'baz', but it is 'bar' right now

The extend method was meant to allow for modifying the core library so you could add transforms and formats and export a new StyleDictionary object for others to consume.

Problem line: https://github.com/amzn/style-dictionary/blob/master/lib/extend.js#L99