adds string/insert

As discussed in #112. 

__Signature:__
```js
insert( str:String, index:Number, partial:String ) : String
```

__Example:__
```js
insert( "matias", 3, "h" ); // "mathias"

// Actual use case:
var path = "images/cat.png"
var retinaPath = insert( path, path.indexOf( "." ), "@2x" ); // "images/cat@2x.png"
```