Hook for Plugins to add commands to Amagaki

[Zoramite]

Would be handy to allow plugins to add custom commands for things that are 'outside' of the normal workflow.

For a translation plugin. You would not want a separate translation request every time a site is build, but being able to manually kick off a translation request would be helpful.

Something like amagaki translate request. Would allow a plugin to process any untranslated strings into a translation request file for the user, store the information in the amagaki cache, etc.

We can use a 'namespace' like amagaki plugin so that it does not restrict the future ability for amagaki to create built-in commands that may conflict with the amagki plugin ecosystem. But it a low probably to need to conflict with a plugin so it was decided to allow plugins use any unused built-in commands. In the future if amagaki wants to expand out commands the plugins will need to update their usage to let amagaki take over the command.