Accent folding / Character normalization

This pull implements the feature requested in #271.

This allows a user to supply a map of characters they wish to normalize for whatever purpose.

```javascript
$('.typeahead').typeahead({
  local: ['cream', 'crème brûlée', 'ça va', 'weißbier', 'Margaret Weiss'],
  charMap: {'àáâããäå': 'a', 'èéêë': 'e', 'ç': 'c', 'ß': 'ss'}
});

// 'c' => ['cream', 'crème brûlée', 'ça va']
// 'cre' => ['cream', 'crème brûlée']
// 'weiss' => ['weißbier', 'Margaret Weiss']
```

Supplying multiple characters in the `charMap` key will map them all to the given value. (e.g. in the above example, any character in the key `àáâããäå` will map to "a".