negate_iife option adds ! to output, that breaks bookmarklet in Firefox ...

By default, in this generator Gulp produces the bookmarklet like:

```javascript
javascript:!function(){ /* ... */}();
```

It breaks bookmarklet on Firefox, because after clicking on it — you are redirected to blank page with 'false' text on it.

So, after my fix

```javascript
gulpUglify({
    compress: { 'negate_iife': false }
})
```

Gulp will produce the output that is understood not only by Chrome, but by Firefox too:

```javascript
javascript:(function(){})();
```

Regards,
Yaroslav.