Feature request: Support vendir to update a whole set of dependencies

When having:
```
- path: k8s/config
contents:
- path: A
git:
url: https://github.com/config/A
ref: master
- path: B
git:
url: https://github.com/config/B
ref: master
- path: k8s/examples
contents:
- path: A
git:
url: https://github.com/example/A
ref: master
- path: B
git:
url: https://github.com/example/B
ref: master
- path: C
git:
url: https://github.com/example/C
ref: master
```

If one wants to update only examples, it would need to do it one by one with `vendir sync -d k8s/examples/A` then B and then C. It would be ideal to support the whole tree under examples when done via `vendir sync -d k8s/examples`.

And linked to #11 It would be ideal to support this use case for -d that doesn't save vendir.lock file as well as with an option to save the vendir.lock file.