Added support for specifying rename of directory when using 'dir' for Output task

To improve the [Output task](https://github.com/mhoyer/albacore/wiki/Output-Task) I added the `:as` option for `dir` configuration. Thus, a directory can now also be renamed on copy - similar to `:as` option in `file` configuration method.

Checkout the [Output wiki page diff](https://github.com/mhoyer/albacore/wiki/Output-Task/_compare/77ed89440ed76d1150b2f644beee7cea46c64c34%5E...77ed89440ed76d1150b2f644beee7cea46c64c34) for documentation update.