Load addon components by convention

[edalzell]

Right now addons have to explicitly define their resources, like tags, command, fieldtypes etc.

In Laravel, there are many places where code is auto-loaded "by convention", if the files are in the right spot and named appropriately.

I always forget to "register" my addon tags, would be lovely if all tags in the addon/Tags folder were automatically registered. Similarly with commands, field types, listeners, essentially anything that's a simple array of classes.