Update magicsuggest.js

Hello, 

I would suggest an improvement to allow customizing the result of ajax. I have a service that returns json text no. 

To solve my problem, I implemented this change: 

added the default parameter: 
```
formatResultAjax: function (asyncData) {
                 return JSON.parse (asyncData) 
             } 
```
And the success of the ajax function _processSuggestions modified the line # 797: 

```
json = typeof (asyncData) === 'string'? JSON.parse (asyncData): asyncData; 
```

to: 

```
json = typeof (asyncData) === 'string'? cfg.formatResultAjax (asyncData): asyncData;
```