Add custom normalizer

Example use case (adapted from Mixpanel integration):

```js
var traits = {
  '$created': 2014,
  'createdAt': 2014,
  'created_at': 2014,
  'created': 2014
};

// Doesn't strip out `$`
var normalize = function(path) {
  return path.replace(/[A-Za-z0-9\.\$]/g, '').toLowercase();
};

// Want to keep only dollar-prefixed version
['createdAt', 'created'].forEach(function(key) {
  objCase.del(traits, key, normalize);
});

// traits => { '$created': 2014 }
```

Allow Mocha GREP

Move normalizer to options param

Update docs with new options param
Merge pull request #11 from segmentio/feature/custom-normalizer

Feature/custom normalizer