Feature request: copy files between releases

I would like to add an additional feature/setting to supplement the `linked_files` and `linked_dirs`.

For dependency management a deploy is significantly sped up if the dependencies are copied between releases (specifically talking about npm/composer here). Symlinking dependencies is not sufficient as it can complicate a _rollback_ and a dependency update could break the live release during a deploy.

Things I haven't included/need work
* Namespace, task name and config variable name
* Removing target files if they already exist (as linked files does)
* Tests
* last_release could be added to `dsl/paths.rb`?

I originally opened this PR, capistrano/composer#9 - it was suggested this might be better as part of capistrano core. I'm still relatively new to ruby/rake so I don't know if my example is the best.