Subcommand plugins via `cocoapods_plugin.rb` file

Any CLAide Command can load subcommand plugins by setting the Command.plugin_prefix for the
command. Installed gems with a file named `<plugin_prefix>_plugin.rb` will be loaded
via that file. Convention is to require the actual command implementation in
the plugin file.

If Command.plugin_prefix is not set, no attempt is made to load any plugins.
Merge pull request #3 from leshill/plugins

Subcommand plugins via `cocoapods_plugin.rb` file