Improvements from Mike

1. Config option to specify port on which to run.
2. New command 'whatcanibuy' which converts your dogecoin balance to multiples of our saved units so you know what you can buy.
3. Improved documentation.
If you don't mind, I'm going to move this under the webhook section below. Since we already have `CONFIG` loaded in `lib/dogetip-slack.rb`, I think we can bypass using `Sinatra::ConfigFile`.