New command is now `kubectl diff` rather than `kubectl alpha diff` since
it's moving out of alpha soon, and will be using dry-run apply to
produce the diff rather than the custom merge logic.
The current interface is kind of clunky and not super easy to use, since
you have to specify parameters to specify which versions to diff. Also
the default isn't the most useful setting.
Change the interface by removing all the parameters and force only one
useful use-case, that is: diffing what's currently live against
what would be live if applied.
Since we're saving nil in an interface rather than the implementation,
we can't compare to nil to check if the remote object exists or
not. Change the struct to save in the implementation.
