Fixed parameter override problem

Fixed parameter override problem
==========================

I have images added some parameters for creating thumbnails like following sample code.  

```
<img src="/img/foo.png?_ex=120x120">
```

But grunt-cache-bust plugin remove the parameter like this.

```
<img src="/img/foo.png?0e981b41a61cfb73">
```

So I modified source for avoiding this problem.  
If filename has parameter already, the plugin added '&amp;' string instead of '?' string.  
And now, I used ```__cachebust_string__``` as parameter name.  
But I don't know this name is good or not.  
Anyway, builded result will be changed like follwing after this fix.  

```
<img src="/img/foo.png?_ex=120x120&amp;__cachebust_string__=0e981b41a61cfb73">
```