Add :skip_spoof_detection option

I had problems with spoof detection and it seems that in order to disable this feature monkey patching needs to be used (eg. https://github.com/thoughtbot/paperclip/issues/1429, http://stackoverflow.com/questions/22527092/paperclip-is-not-supporting-doc-file). I think that there should be a better way, so I've added :skip_spoof_detection option.

Example:
```ruby
class User
  has_attached_file :attachment, skip_spoof_detection: true
end
```

I'm not sure if it is the best solution, but I think that there should be such feature available, because currently spoof detection is not very reliable.