Make delete, diff, show, update, and validate aware of ksonnet apps

The ksonnet.next design doc specifies the core kubecfg verbs (i.e., the
ones listed above) to all have the form:

  kubecfg <verb> [<env-name>|-f <file-or-dir>]

That is to say, each of these should be able to take either an
environment name, or a `-f` flag with a list of files and directories to
apply `verb` on. In the case of the environment, we will apply `verb` to
every component in the `components/` directory.

This commit implements this behavior for all these verbs.