Add --watch support

[howardjohn]

It would be awesome to deploy some big resource (say Knative Service) and then kubectl tree --watch it to see how things get ready.

The alternative would be watch kubectl tree, but then we lose colors. So maybe an alternative would be a way to always have colors, even when not a TTY?

[ahmetb]

We could, but I am not sure if the benefits outweigh the effort to get this done. As you said, the watch command can be an easy hack.

FWIW it's "us" disabling the colors when the command is piped or executed with a stdin that's not a tty. So maybe we can allow something like --force-colors and you can use watch --color.