Use `cocoapods` as CLAide#Command.plugin_prefix

To create a CocoaPods subcommand plugin gem, add the file
`lib/cocoapods_plugin.rb` and load your subcommand implementation from there.

When the `pod` command runs, it will look for that file in your gem and load
it before parsing the command line.

Use this PR with https://github.com/CocoaPods/CLAide/pull/3 .