Support various manifest sources in `apply`

`skaffold apply` current accept one or more manifests passed as arguments, but only local files can be passed. we could enhance this by supporting

* `glob` arguments akin to the `kubectl` deployer (`skaffold apply manifests/*.yaml`)
* URLs to remote manifests (`skaffold apply https://raw.githubusercontent.com/GoogleContainerTools/skaffold/master/examples/getting-started/skaffold.yaml`)
* manfiests passed directly to `STDIN`, similar to `kubectl` (`skaffold diagnose --yaml-only | skaffold render -f - | skaffold apply -f`)