Parsing suggestions before displaying them

This change adds `suggestionsCallback` option which lets you parse suggestions before displaying them in the browser. For example, you can sort suggestions based on datum properties.

Example usage: sort suggestions by `count` property:
```js
$('input').typeahead({
	local: [
		{ value: 'Sales', count: 57 },
		{ value: 'Server problem', count: 5 },
		{ value: 'Support', count: 251 }
	],
	suggestionsCallback: function (suggestions) {
		// sort suggestions by "count" property
		return suggestions.sort(function(a, b) {
			return b.datum.count - a.datum.count;
		})
	}
});
```

This example returns suggestions in the following order:
```
Support
Sales
Server problem
```