Add an --only-plugins and --only-themes config flag

[Orrison]

Today we have a --skip-plugins and --skip-themes flag that allow us to specify what plugins and themes NOT to load. But sometimes you want to load one or more plugin/theme but none of the others. Perhaps a plugin exposes some wp-cli command you need to use but you don't want any other plugins loaded while running it.

To do so now would take some run around to get a list of plugins then pass them to the --skip-plugins command sans the plugin you actually wanted loaded.

[danielbachhuber]

Thanks for the suggestion, @Orrison !

--skip-themes and --skip-plugins are global parameters, and global parameters can conflict with plugin parameters. As such, I think we're unlikely to introduce --only-plugins and --only-themes global parameters.