Output css file to dedicated folder (outside bundle sources)

If the assets have been installed using the symlink option (which is the default), outputting css files into *web/bundles/somebundle* means outputting them directly into the *Resource/public* directory of the bundle, which can cause permission issues as the vendor directory isn't supposed to be writable.

This PR redirects the output to a dedicated *css* folder to avoid that (similar changes are coming in the core bundle).

PS: I would be nice if you could create a tag (last one is pretty old anyway).