Support calculation md5 / sha1 hash of each uploaded files

Sometimes it is nice to have file's md5 or sha1 after upload. I tried to do it by using req.file().pipe but found it's extremely hard. So I modified skipper-disk package. When giving an option
```
{
  hash: 'md5'
}
```
In the returned results, it will show file's hash for example:
```
[ { fd: '/srv/http/.tmp/uploads/d3cdcbd2-5b6a-4131-b9ce-2a5da00d4f9d.ini',
    size: 115,
    type: 'application/octet-stream',
    filename: 'desktop.ini',
    status: 'bufferingOrWriting',
    field: 'files[]',
    extra: { md5: '359bb6716c7fb00e02d4b681a0bce711' } } ]
```

if `hash` option is not given, it will load nothing. Sinece it's using a pipe chain, the overhead of hash calculation is minor.