Possible switch/config option to allow helm lint with "--strict"

There should be a switch or a config option that allows for the lint to use `helm lint --strict` to ensure a failure if there are missing values.

https://github.com/helm/chart-testing/blob/master/pkg/tool/helm.go#L53

Perhaps even just changing the line to
```golang
return h.exec.RunProcess("helm", "lint", "---strict", chart, values)
```