Create an empty file

In some of my tests I need to create an empty file. I think it's more readable to have `#empty_file('name')`

```
slide1 = instance_double('Middleman::Presentation::NewSlide')
allow(slide1).to receive(:path).and_return empty_file('01.html.erb')
```

Than having `#write_file('name', '')`
```
slide1 = instance_double('Middleman::Presentation::NewSlide')
allow(slide1).to receive(:path).and_return write_file('01.html.erb', '')
```

I wrote a wrapper for that, but maybe you find it worth to include it in aruba.