Allow for file sources array without destination

It's sometimes desirable to only provide the paths to sources (for example, if they are to be overwritten).

This PR allows for a simplified syntax when a plugin would like to do this.

```js
task : {
    files : ['path/to/source1.js', 'path/to/source2.js'],
},
```

Instead of the other two commonly used approaches : Accessing the files data through `this.data.files`, or by making users provide the same path for both source and destination.