Provide check duration time in json output

Sample json:

"Durations": {
    "admission-controller-webhook": "5.193µs",
    "bare-pods": "12.363µs",
    "default-namespace": "31.826µs",
    "fully-qualified-image": "224.87µs",
    "hostpath-volume": "86.482µs",
    "latest-tag": "158.023µs",
    "node-name-pod-selector": "9.108µs",
    "non-root-user": "44.107µs",
    "noop": "310ns",
    "pod-state": "9.036µs",
    "privileged-containers": "56.992µs",
    "resource-requirements": "97.013µs",
    "unused-config-map": "149.937µs",
    "unused-pv": "33.115µs",
    "unused-pvc": "49.56µs",
    "unused-secret": "139.136µs"
}

@adamwg - last Friday we identified that the API requests are the reason for the tool being slow. Do we want to maybe wait on https://github.com/digitalocean/clusterlint/pull/48 for some perf tests while we assess if the PR is really required?

digitalocean / clusterlint

Provide check duration time in json output #49