Improve Organization of Flags for Sign/Verify?

[tri-adam]

As mentioned by @fnikolai in #1095 (comment), it might be useful to organize the increasing number of flags in singularity sign and singularity verify:

Perhaps something along these lines?

$ singularity sign --help
...
PGP flags:
      --whatever string                                                                                                                                                                                                      
                Some Whatever message Here
      ....

X509 flags:
      --certificate
                Path to x509 certificate
      --key
                Path to x509 key
      ....

There is something called 'NamedFlagSets' that could help us with grouping the flags.
spf13/cobra#1327