Improve creating of releases

Get `hub release create --attach A_FILE` and `hub release create --attach A_FOLDER` to work. I reworked @calavera's previous changes https://github.com/github/hub/commit/c20cdc40001c74bb3d0b4a805c66ecdd3a2449c6 to make it more readable.

It prob. make sense to add some cucumber specs as well.