Allow Command object to skip parser.set_defaults()

This commit is necessary to support subcommands via yaclifw. If subparsers are
added to the Command parser but parser.set_default(func=xx) is called, the
top-level parser default function will be used even if subparsers define their
own default function.
Merge pull request #1 from sbesson/fw-callback

Allow Command object to skip parser.set_defaults()