Schema cache

Each time `kubeval` is executed are the k8s schemas downloaded from https://kubernetesjsonschema.dev. It would be great if the fetched schemas could be cached to disk.

The cache could be cleared using a new command: `kubeval purge cache`. The usage of the cache could be disabled by passing a flag: `--no-cache`.

```
cat manifest.yaml | kubeval --kubernetes-version 1.16.0 --no-cache
```

The cache could be stored inside the `$HOME` directory of the user or inside a specific location. `kubeval` should not fail when unable to write the cache to disk (a warning instead could be logged).

This would be very useful for me personally since I am using `kubeval` inside my git hooks to validate my project HELM charts.