Improve README and cli user feedback

- Don't add a todo that is an empty string, just abort instead.
- When adding a todo, display a message to say it was successfully added.
- Pressing spacebar will no longer throw an error in the delete todos when there are no todos to delete. Instead a message will be displayed saying there are no todos.
- logging saying all completed todos were deleted when they were.
- README fix.
Oh I see. The space between ticks and language abbrev. doesn't make it render different. See https://github.com/ryanseys/gcloud-node-todos/blob/rs-improve/README.md

I think the fix would be adding a new line before the ticks. Good catch on this though, I never noticed. Dang markdown!