Add support for customizable filenames

Far from perfect at the moment, but it works. Not sure how we're going to deal with the `-` in cropped. If we're setting it in the template there will be a empty `-` at the end of files that aren't cropped.

Fixes #35.
actually, this is misleading as it's not the hostname.

```sh
pageres http://yeoman.io/learning/ 1000x1000
```

results in:  `yeoman.io!learning-1000x1000.png`