Fixed mutation of the WOW class' config

When create two (or more) instances of the WOW with different configs, all instances will be created with same configs.

This test can explain this bug:

```CoffeeScript
it "creates two instances of the WOW.js with different configs", ->
  wow1 = new WOW
    boxClass:     'block1'
    animateClass: 'fancy1'
    offset:       10
  wow2 = new WOW
    boxClass:     'block2'
    animateClass: 'fancy2'
    offset:       20

  expect wow1.config.boxClass
    .toBe "block1"
  expect wow1.config.animateClass
    .toBe "fancy1"
  expect wow1.config.offset
    .toBe 10
```

It will not passed.
This is not really the same behaviour that @matthieua intended to do with the `.extend(…)`. Namely, the reason we merge the config with the defaults is so that you can specify only some options, but still get the rest of the defaults.