add -d flag for clone to owner/name directory

Adds a -d flag to the clone command for automatically cloning to a nested directory "owner/name" matching the GitHub username and project name.

e.g.
```sh
$ hub clone -d defunkt/hub
```
which clones to "./defunkt/hub" instead of the default "./hub".

I use this pattern myself anyway for all my GitHub clones; I wanted to make it convenient.