Load subcommand plug-ins at runtime

A subcommand plug-in is a gem with a file that can be required at

    <command-path>/<command-name>

The `command-path` is the namespace of the command converted to a path. For
example `Pod::Command` becomes `pod/command`. And an `open` subcommand would
be required at `pod/command/open`.

The first working plug-in is [here](https://github.com/leshill/open_pod_bay) and provides an `open` subcommand to open a workspace. `gem install open_pod_bay` to try it with this PR.