Add nerdctl support

[darktohka]

Right now, there is no support for nerdctl, a pretty nifty container CLI for containerd: https://github.com/containerd/nerdctl

This PR adds support for a new image source: --source nerdctl

This image source is supported on all OSes, nerdctl has support for Linux, Windows and Darwin as well.

Nerdctl saves images using nerdctl image save <id>. If this errors (the image may not exist), nerdctl pull <id> is tried to pull the image if possible.

Nerdctl can also build images the same way as the Docker CLI (nerdctl build --iidfile)

[polarathene]

You may need to rebase your PRs due to the recent PR syncing a fork back to dive, maintainer of dive is active again on the project so this would be a good time to have your PRs rebased for review.

[wagoodman]

Thanks for the contribution! I'm all for supporting containerd, though I'm about to put in a refactor code responsible for fetching images to use anchore/stereoscope in #569 , which already has support for containerd, so it might not be worth the time trying to rebase this PR yet. I'll close this PR in conjunction with the new PR that will bring in stereoscope.