add an option to display a bot deployment command

for those who use bots for deployment, this will be a handy helper that will print out the command you can send the bot.

to enable, you will need to set `bot_deployer_command_formatter` in your deployment file (e.g. `deploy.rb`). see the example below.

```ruby
set :bot_deployer_command_formatter, "!deploy web %s to %s"
```

alternatively, you can use it to format any command or message you would like using the `new_tag` and `stage` variables which are defined in `:cut_tag` task.

- bumps version to `0.1.1`

(cc @steverice @esigler)