Add ability to specify different resolutions for different URLs

CLI usage: 
```bash
pageres [yeoman.io 1024x1211 499x122] [todomvc.com 1324x2321 980x667]
```

API:
```js
var items = [{ 
	url: 'yeoman.io', 
	sizes: ['1024x1211', '499x122']
}, { 
	url: 'todomvc.com',
	sizes: ['1324x2321', '980x667']
}];

pageres(items, null, function (err, stream) {
	...
});
```