Making `pod plugins` an abstract command with search as default subcommand

* `pod plugins` is now an abstract command (fixes #11)
* ~~`search` is now the default subcommand (fixes #12) and accepts an empty query (making it list all the plugins)~~ _[EDIT] See below, now the new `list` subcommand is the default one_
* Some refactoring has been done, especially splitting classes and bacon specs in separate files (fixes #10)
* Updated specs accodingly