Flexible shared assets prefix

Hi there,

Rails allows us to customize assets prefix, so does capistrano. However, shared assets prefix is hardcoded in all the tasks in `lib/capistrano/recipes/deploy/assets.rb`.

Under some circumstances, `shared/assets` might be already taken and capistrano users won't be able to use out-of-the-box assets pipeline scripts due to that simple reason.

The workaround is to override every single task that works with shared assets folder, but this is a lot and capistrano users have to maintain their own version of almost every assets precompile task.

This pull request simply adds another variable `shared_assets_prefix` with the default of "assets" which is backward compatible and adds more flexibility for such rare, but feasible case.

Please let me know if you have any questions of concerns.

Thank you.