Basic schema cache

[johanneswuerbach]

When passing in multiple files each schema is downloaded multiple times.

This PR introduces a simple in-memory cache to download required schemas only once during a validation run.

It uses the approach described in https://github.com/xeipuuv/gojsonschema/blob/master/README.md#validation to share load the schema only once.

This reduces the validation time of our ~1k yaml files from 5 mins to ~10s.

[garethr]

@johanneswuerbach this is great, much appreciated. I'll try this out and get a new release of kubeval out with this. Thanks for the functionality and the tests too.