Pass #url `args` to #default_url

This will give us a few extra pieces to work with to name
a certain thumbnail for something. For instance:

```ruby
def default_url(*args)
  vname = version_name || (args.first if versions.key?(args.first))
  [model.mounted_as.to_s, vname.to_s, "default.png"].join("_")
end
```

This will allow us to call `model.attachment.url(:not_a_version)` and still
return `"attachment_not_a_version_default.png"`, which is useful in cases of
conditional versioning.