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.

> 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