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
  [model.mounted_as.to_s, vname, "default"].join("_")
end
```

This will allow us to call `model.attachment.url(:not_a_version)` and still
return "attachment_not_a_version_default", which is useful in cases of
conditional versioning.