Add file md5 check support

Maybe you want to support
```
"""
<hash>
"""
```

here as well. But then you would need to change

```
Then(/^the file "(.*?)" should have a fingerprint "(.*?)"$/) do |file, exact_hash|
```

to something like that (not tested):

```
Then(/^the file "(.*?)" should have a fingerprint(?: "(.*?)"|:))$/) do |file, exact_hash|