Convert concat blocks to non simple form

I'm working on a couchdb app. They need files outputted to _attachments directory. I found that after setting up grunt-usemin I couldn't get "grunt useminPrepare concat" to work properly.

After a lot of digging. I found documentation that grunt ignores sub/multi tasks starting with _ in them. This means outputting to the _attachment directory fails.

So simple patch to prevent _ in the task name by defining the more complex block.