kitectl: Reflect the changes that are mentioned in the PR.

- Do not stutter by appending `Command` to every subcommand structs.
- Remove version command since the new CLI package already provides this
  functionality.
- Add a default.go file that shares the default objects such as
  Default CLI UI and Default Kite Client.
- Remove population of the commands in main.go. Move this construction into the related subcommands.
- Test kitectl in Makefile.