Added support to set which subreddit to fetch from

Hi Bill! 
I really like the idea of the application, and made some modifications to it.

- Deddit was missing a dart:async import
- Renamed `example` into `bin` which is recommended for cli apps
- Added some argument parsing to fetch other subreddits

I'm sending a PR just to show you what I did, feel free to merge it if you like. :smile: