Added option to provide more than one convert command per target task

I had multiple files to convert, each with its own options, and it seemed illogical to create target for each conversion, so I created this fix.
You can still do:

```javascript
"imagemagick-convert":{
  dev:{
    args:['test/resizeme.jpg','-resize', '25x25', 'test/resized/resizeme-small.jpg']
  }
}
```

but you can now also do:

```javascript
"imagemagick-convert":{
  dev:{
    args:[
      ['test/resizeme.jpg','-resize', '25x25', 'test/resized/resizeme-small.jpg'],
      ['test/resizeme.jpg','-resize', '15x15', 'test/resized/resizeme-tiny.jpg']
    ]
  }
}
```

(also replaced some tabs with spaces)