Add support for plugin directories

[Schamper]

General idea is that you create your plugin in a file called _plugin.py in a certain directory. That directory can then hold other files such as specific helpers. The plugin path/name is then derived from the directory path up until the module name, instead of including it.

For example:

apps/something/myapp
- _plugin.py
- helpers.py

The plugin will register under the apps.something.myapp path.