Support calculating md5 / sha1 hash for each uploaded files

[dzcpy]

It is very handful if we could have file's md5 or sha1 after upload, for example, when using it to check duplicated files, as a global unique file name or as database primary key etc.. I tried to do it by using req.file().pipe but I found it's extremely hard without reading through the file again (without hacking the adapter). So I modified skipper-disk package and added this feature. When giving an option

{
  hash: 'md5'
}

In the returned results, it will show file's hash. 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 so there's no perfomance impact when we not using it.
Since it's utilize the same pipe chain as skipper-disk's fs.createWriteStream, the overhead of hash calculation is minimal as well.

[dzcpy]

@caasjj I think you are right, I modified the code again, thanks for the suggestion