Implement stateless flag from the spec

[technosophos]

Add support for stateless:

cnabio/cnab-spec#85

I think this should be fairly straightforward, but probably not easy enough to be a good first issue.

[radu-matei]

cc @simonferquel

[radu-matei]

As initial support for this lands in #630, this brings an interesting discussion regarding the duffle UX:

• according to the spec, [a stateless] action does not act on a claim.
• custom actions are run at this point with duffle run: duffle run ACTION RELEASE_NAME [flags].

This means that we would need duffle run to be able to act both on a release, and on a bundle.

What would be the best way to ensure we keep the command clear?
If we think documenting is enough, I'd also be happy with that, but we need to clearly state how actions can be run with Duffle.

[astrieanna]

This and #830 seem like duplicates.