Add plugin class

Add Hologram::Plugin that other plugins can extend from. This takes
takes care of the following cases:

- Checks arguments to see if the plugin is active (is_active? = true)
- Replaces plugin text when disabled
- Plugins get their text from [[[plugin:plugin-name ... ]]] blocks. This
is done automatically by the base plugin class. The .plugin method can
return either an emtpy string or some other text that will be put in the
plugin block's place.