Load subcommand plug-ins at runtime

We use rubygems to load a subcommand plug-in gem with a file that can be loaded at

    <command-path>/plugin.rb

The `command-path` is the namespace of the command converted to a path. For example `Pod::Command` becomes `pod/command`.

The `plugin.rb` file should either define the subcommand(s) directly or require the files for the subcommand(s). For example, the first working plugin, [open_pod_bay](https://github.com/leshill/open_pod_bay), provides an `open` subcommand and has the following `plugin.rb`

    require 'pod/command/open'

`gem install open_pod_bay` to try it with this PR.