Add `duffle claim show --output`

[astrieanna]

Part of https://github.com/deislabs/duffle/issues/747

After making the code compile against the cnab-go library with outputs support implemented, this PR adds a --output flag to duffle claim show. This allows users to dump the contents of output files to stdout.

• The output flag takes the name of an output as stated in the bundle; by showing a claim the user can view the names of all the outputs that are part of the bundle.
• Because the cnab-go library implementation returns all the outputs that were generated, even if errors occurred during execution, users will be able to see what output files the invocation image produced during failed runs.

cc @youreddy @jeremyrickard

[astrieanna]

@silvin-lubecki does this side of the PR look good?

[radu-matei]

/brig run

[youreddy]

The PR in cnab-go was merged, going to update the Gopkg.toml file to point to the updated version!

[jeremyrickard]

@youreddy should we release cnab-go or are you planning on pinning to a commit #? I'd prefer a released version, but we can always update to that once we have a few more things in.

[youreddy]

@jeremyrickard, I was going to specify master but you're right we should do a released version. And yes to releasing cnab-go!

[jeremyrickard]

/brig run

[astrieanna]

@jeremyrickard /brig run ?

[radu-matei]

/brig run

[jeremyrickard]

@astrieanna sorry for the delayed response. Were you asking me to re-run it, or what "/brig run" does? Happy to explain what it does if you'd like!

[jeremyrickard]

/brig run

[jeremyrickard]

Thanks for the persistence on this @astrieanna and @youreddy, much appreciated.