Allow file sources only array. 

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 : {
        src : ['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.