Add crane's auth command to facilitate logging into a registry

[tcnghia]

Fixes #155

/assign @jonjohnsonjr @mattmoor

[codecov-io]

Codecov Report

Merging #243 (f5ec346) into master (36a921f) will not change coverage.
The diff coverage is n/a.

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  Coverage   37.19%   37.19%           
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  Files          33       33           
  Lines        1460     1460           
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  Hits          543      543           
  Misses        828      828           
  Partials       89       89           

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[jonjohnsonjr]

I would have just snagged the login command, not both, but the cobra doc is currently hardcoded to crane:

$ ko auth login --help
Log in to a registry

Usage:
  ko auth login [OPTIONS] [SERVER] [flags]

Examples:
  # Log in to reg.example.com
  crane auth login reg.example.com -u AzureDiamond -p hunter2

Flags:
  -h, --help              help for login
  -p, --password string   Password
      --password-stdin    Take the password from stdin
  -u, --username string   Username

Easy, hacky solution would be to just replace logincmd.Example with your own hard-coded string.

More involved solution would be to rework NewCmdAuthLogin to figure out what command group it's in and print the right thing. I'm not sure if that's possible.

[mattmoor]

@jonjohnsonjr We could make the function use os.Args[0] 😉

[jonjohnsonjr]

That's not sufficient because for crane that would just be crane login but in reality we need crane auth login, which makes me regret this command group entirely, and I think they should have just been top level.