Changes:
- moved the command names from main to the executor package.
- moved the unknownCommandError to the executor package and renamed it
to UnknownCommandError.
- define the command summaries within the executor package.
- replace the switch statement in main.run() with a hashmap for looking
up the corresponding executor to the given command.
- transformed BlockExecutor to BlockOrUnblockExecutor.
- transformed FollowExecutor to FollowOrUnfollowExecutor.
Additional fixes:
- fixed flag for the version executor.
