Initial implementation of an addons manager

When loaded, will load all auto-load marked UIs.  Shows current UIs and
allows custom UIs to be loaded.
Auto-load Addons

The addons mod is now auto-loaded (by character.ts) and will auto-load
any mods listed in modules-list.js as autoload.
Merge pull request #4 from Mehuge/feature-addons

Feature addons