Changes to plugin base class to accommodate awkward developers who use symlinking to include plugins in their setup; now allows us to pass an explicit `$type` into the `setup` method. Remove lots of `error_log` calls. Add `sil_plugins_dir` and `sil_plugins_url` filters, to allow placement outside the `wp-content/plugins/` folder, for example using `require_once` to include from the theme `functions.php`. Will be useful for deploying in Automattic VIP.
Show a table of all available languages, with form controls to: * Select languages to make active * Select whether a language is RTL or LTR * Also add (stolen) code to translate from language code to language name. Also add my plugin base class to the project and extend from it for the Languages class.