Include I18n::Backend::Pluralization for greater flexibility.

Non-english languages can add lang/<lang>.rb alongside a yaml file to
define the language's pluralisation rules. See for examples:
https://github.com/svenfuchs/i18n/blob/master/test/test_data/locales/plurals.rb
Improvements to the plugin loader

It will now destroy as many objects as possible when performing a full
reload, and will look for plugin files on disk to #load when a constant
is not already defined. This makes it possible to load entirely new
plugins without restarting a bot.

Additionally '!reload loader' will now work.