Add `--patch` flag to apply a Ruby patch from stdin

Example:

```sh
curl http://patches.com/ruby.patch | rbenv install -p 1.9.3-p448
```

The only possible way to pass a patch is STDIN. I think that's OK, because it makes arguments processing easier, and allows the user to pass in multiple patches by just concatenating them through STDIN.

/cc @sstephenson