Use files.src in replacement of files.data/template, to allow globbing (and more)

To define the list of files to compile (data+template), I propose to only set one of them, and let the second one use the src property.
That way, we can use all the [powerful files functions from Grunt](http://gruntjs.com/configuring-tasks#files), like globbing.

For example, we can compile all templates in a directory without listing them:
```js
twig_render: {
    dist: {
        files: [{
            data: '<%= config.app %>/data.json',
            expand: true,
            cwd: '<%= config.app %>/views/',
            src: ['**/*.twig', '!**/_*.twig'], // all templates except partials
            dest: '.tmp/',
            ext: '.html'
        }]
    }
}
```