Provide an --exclude command line argument to exclude some patterns or files

Hey, kubeval looks really useful. We're adding it to our heptapod/gitlab CI. It's as simple as

```
kubeval:
image: garethr/kubeval
script:
- kubeval --directories .
```

We use kustomize https://kustomize.io/ so we have a bunch of `kustomization.yaml` which fail the test. Since we also store the resulting generated files in our repos, we'd like to be able to run `kubeval --directories . --exclude kustomization.yaml`